Cohort profile: The Bristol IVF Study- A longitudinal study of women, their partners and treatment outcomes following assisted reproductive technologies

The BRI ST-IV F c ohor t enables nov el r es ear c h in t o the pr edict or s and c ons eq uen c es of AR T c once p tion, with comparison t o a na t ur ally c onceiv ed cohor t, the second g ener a tion of the A von L ongitudinal S t udy of Par ent s and Childr en ( ALSP AC-G2). List of Abb r evia tions: ART : as s is t ed r epr oductiv e t echnology , IVF: in vitr o f e rtiliz ation, ICSI: intr a cyt oplasmic s per m injection, BCRM: Bris t ol Centr e f o r Repr oductive M edicine, EDT A: E thylenediaminet et r aacetic acid, ACD : ac id citr at e de x t r ose, BMI: Body ma s s inde x Key wo rds : in v itr o f er tiliz a tion, as si st ed r epr oduc t iv e t ec hnolog ies, longitudinal s tudy , pr egnanc y out c omes . CC-BY 4.0 International licenseIt is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. (which was not certified by peer review) The copyright holder for this preprint this version posted April 25, 2025. ; https://doi.org/10.1101/2025.04.24.25326332doi: medRxiv preprint 4 Backgr ound Inf er tility , as de fined by t he W orld Health Or g aniza tion (WHO), is the inabilit y t o achieve pr egnanc y a f t er 12 months or mor e of r egula r , unpr otect ed se xual int erc our s e. It af f ec t s r ough ly one in s ix c ouples globally , w it h lif etime pr ev alence es tima t es r anging fr om 4% t o 40%, d e p ending on r egional and me tho dological v ar iat ions (1). Althou g h mos t pr ev alenc e da t a come s from h igh inc ome co u n tries, es tima t es fr o m low- t o mid dle -inc ome c oun tries sug ge s t t hat r a tes ar e c ompar able (1). Be y ond it s biologic al implica tions , inf er tility has pr of ound ps y chologic al and s ocial e ff ects , oft en leading t o emo tional dis tr ess, ment al health challenges, and r educed pr oductivity (2). Ass i s t ed r epr od uc t ive t echnologies (AR T) ar e eff ectiv e m edic al int erve n tions f or tr eat ment of inf e r tility . AR T r e f e r s t o all int erv e n tions tha t in c lude t he in vitr o handling of bot h human oocyt es and sper m or embry os f or the purp ose of r eprodu c t ion (3) . The most c ommonly used f orms of AR T ar e in -vit ro f ertiliza t ion (IVF) an d IVF with In t r ac y t oplas mic sperm injection (ICSI) . Pla ns f or dev elopin g the BRIST-IVF c ohor t beg an pr ior t o int ern a tiona l gl os s ar i es clar i f yi ng t er ms f or i nf er t ilit y an d i t s t r ea t m en t . WE ha ve r et ai ned t he s t udy name (BRI S T- IVF) and us e ART as a g ener al t erm f or IVF and ICSI in this paper . Glo bally , AR T h a s r e sult ed in the b ir th of o ver 8 million individua ls, wit h the number of A R T cy cle s c on tinuing t o rise in high inc o me c oun tr ies , wit h incr easin g u se in low- a n d middle- inc ome c oun tries (4) . Iden tifying modifiable risk f a c t or s tha t inf lu enc e live birt h succes s and of f spring a n d pa r ent al hea lth p os t concept io n is es sen tial f or impr o v ing tr ea tment e ff ic acy . His t orically , r es ear c h o n f a c t or s as so c i a t ed wit h AR T out c omes has f o c u s ed on clinic a l f act or s tha t ar e r outinely collect ed dur ing IVF tr eat me n t including ma t er na l ag e, r epr od uc t ive his t ory a n d type of IVF tr ea tmen t (5, 6). However, incr ea sing evidence s ugge st s that ART . CC-BY 4.0 International licenseIt is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. (which was not certified by peer review) The copyright holder for this preprint this version posted April 25, 2025. ; https://doi.org/10.1101/2025.04.24.25326332doi: medRxiv preprint 5 outcome s ar e als o in fluenced by mater nal and pa t ernal h ealth and lifestyle chara cteristic s . For example, s t udies have demonstr a t ed that bot h ma tern a l and paternal s mo k ing are asso ciated with r educed liv e birth rates following A R T (7, 8) and that o v erweigh t or o b es it y in bot h th e fe ma le and ma le par tners is a sso ciated with r educed lik elihood of succes s in IV F tr eatmen t ( 9, 10). Higher body mass in dex ( BMI) ma y influence s uc ce ss of I VF treat me nt via met abolic alterations t hat affect b oth oocyte and sperm q ualit y . In w o men, metabolic pro files in blood have been shown to ass o ciate w it h the c o mposit ion o f the follic ular fluid ( FF ) , the microenviro nm ent surr ounding th e matur in g ooc yt e (11). Stud ies in bot h animals and humans have s ho w n that FF meta bo lit es ar e assoc iat ed wit h oocyte quality and with pregnanc y ou tcome following ART (12, 13). Similarly, in males , metabolit e prof iles in both serum and s emin al fluid have been s hown to r elate to male in fertility (14, 15). Advancements in h igh t hroughput n uc lear magnet ic resonance (N MR) spec t ros c o p y now allow prof iling o f over 200 c ir c ulat in g metabolites in blood s er um or pla sma ( 16). As s e s sin g pre- tr eat men t and/ or in treat m ent metabolite prof iles may help ide nt ify biomar k er s pred ictive of IVF s u c ce ss and un cover pot ential pathways for int ervention. A recent study of 400 w o men and their male par tners undergoing IV F tr eatment in Glasgo w, UK, identified sev er al novel associations between s erum metabo lites and markers of ova r ian reserve and sperm parameter s (17, 18) . Thes e f indings highlight the need for f urther r eplic at ion and pros pective studies t o es t ablish met ab olic pr edic t ors of s ucce ss. As liv e-birth suc ce ss r at es f ollowing AR T c on tinue t o impr ov e, gr eat er at t ention has been placed on pot en tial per inat al and lo ng-t er m health ou t comes of AR T-c o nceived off spring. Man y s tudies e xamining t hes e out comes ha v e been desc r iptiv e, f ocus ing e x clu s iv ely on A R T- c onceiv ed individuals without c o mpar is on gr ou p s or u sing select ive pee r- bas ed . CC-BY 4.0 International licenseIt is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. (which was not certified by peer review) The copyright holder for this preprint this version posted April 25, 2025. ; https://doi.org/10.1101/2025.04.24.25326332doi: medRxiv preprint 6 c omparisons. R e cen t r esults fr om a lar g e c ollabor at ion of b irth c ohor ts , wher e A R T and na tur ally-c onceiv ed individuals wer e f ollowed -up ide n t ic ally f r om p regnanc y t o ear ly adultho od f o und lit tle evidence tha t AR T c onc ep tion a f f ec t ed bo d y c omposition or c ar diomet abo l ic health u p t o early adulth ood (19). These findings wer e further s uppo rt ed in a lar g e U K elec t r onic health r ec or d s tu dy in whi ch bo th c on v en tional po pula tion a n aly ses , and w it hin sibling analy s es, w hich c on tr ols f or f amily level c onf ounding , s uc h a s par ent a l inf ertility , f ound no a ssocia tion bet ween A R T concept ion and h o s pit al a d mis sion s f or cir cula t ory dis ease s (20). Elec t r onic health r ec or d s tudies ha v e also be en used t o e xplor e e ff ects of AR T c onception on perina t al ou t comes . R ec en t analyse s i nco rpor a t in g both c on v en tio nal po pu la tion a n d with in siblin g c omparison s, support a caus al ef f ec t of fr esh- embr yo A R T t r ans f er c ompa r ed t o na t ur al con c ept ion on an incr ea sed risk of small f or g est at ional ag e, while fr o z en embry o tr ansf er incr eas ed the r i s k of la r g e f or ges t a tion a l a g e c ompar ed t o natur al c onception. Bo th fr esh and fr o zen embry o tr a n sf er in c r ea sed th e risk of pr et erm birth (21) . W h ile bir th c oho rt and electr on ic health rec or d st udie s p r ovide v a luable insight s int o tr ea tm ent-r e la ted dif f er ences (e.g ., IVF vs. ICSI, f r esh v s . f r oz en embr yo tr ansf er ), t hey c annot ev alua t e m or e det ailed tr e a tmen t v ariables , su c h a s ov arian s timula tion pr ot oc ols . Furt hermo r e , elec t r onic healt h r ec or d st udies c annot e xplore pot en tial molecular mechanisms un der lying AR T-r e la t ed out c omes . The BRIST-IVF co hort, with its det ailed me t abolic and clinica l da t a, is well-positioned t o addr ess t hes e g a ps . The Br is t ol IVF Stu d y ( BRIS T-IVF) wa s d es igned t o es t ablish a clinic al AR T co hort t o under t ak e r esear ch o n th e pr ed ict io n and cau s es of live birt h succ e s s, pr egnan c y c omplic a tions and perinat al out c omes. The initial r ec r uitmen t t ar g et wa s 1,20 0 coup le s or single women. This was amende d as a r e sult of the COVID pandemic (det ails below ) . CC-BY 4.0 International licenseIt is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. (which was not certified by peer review) The copyright holder for this preprint this version posted April 25, 2025. ; https://doi.org/10.1101/2025.04.24.25326332doi: medRxiv preprint 7 The initial objectiv es (pr oposed in 2019) wer e t o : 1. Impr ove t he ac cur acy of pr edic t ion of live bi r th suc c es s with ART; 2. Identif y modifiable risk fa c t ors and mechanis m s that might be t ar gets for dev eloping inter v entions (lif es t yle or c linical) that could improve rates of liv e bir th suc c es s; 3. Det ermin e the as so ciat ions of ART and different ART tr eat men t proto c ols o n fet al epigenetic s ignal s ( s pecifically cor d-blood DN A Methylation and potentially his t one marks); 4. Det ermin e th e impac t o f ART and different A R T t reatment prot ocols on cardio- metabo lic risk factor s (and their tr a jec t ories) in mot he rs, and part ners dur ing t he f i r s t 5 years after t reatmen t; 5. Det ermin e the impac t of ART and different AR T tr eatment p r otocols on maternal, part ner and o ff s pr ing mental health. Du e t o t he Covid-19 p ande mic, BRIST-IVF was clos ed f or a period o f 13 mon ths. It became clear f ollowing the closur e, and t he e f f ect of the fir st clinic vis it s f or tr ea tment being online, r a ther than in per s on onc e the A RT clinic r eopened, tha t it wa s unlik ely tha t t he initia l r ecruit ment t ar g et would be met. As a r esult, t he st udy wa s c losed earlier than p lann ed as part of the NIHR r esear ch and r ec over y r eset pr ogr amme (22). The spe c i fic imp a ct s of the pandemic on s tudy oper a t io ns ar e su mmar ised in Bo x 1. The objec t ives h av e been r ev ised t o r e flec t t he numb er of par ticipan ts r ecru it ed and the dur a t ion o f f ollow-up and we will no w f oc u s on two br oad objectiv es : 1. To investigate deter minan ts of live bir th ra t es f ollowing ART. This will includ e tr eatmen t f ac t ors (e.g. p rotocols) a nd biologic al measure s (e. g. N MR metabolite measures). . CC-BY 4.0 International licenseIt is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. (which was not certified by peer review) The copyright holder for this preprint this version posted April 25, 2025. ; https://doi.org/10.1101/2025.04.24.25326332doi: medRxiv preprint 8 2. To compar e pregnanc y and p e r inatal out c omes between ART pregnancies a n d non- ART pr egnancies usin g the Avon Longitudinal Stud y of Parents and Childr en (ALSPAC) G2 c oh ort ( 23) a s a comp aris o n group, and e xplor e the extent to which any of t hes e differences ar e mediated by N MR met a b olites , plac ent al tis s ue D NA methy lat ion or tr ans cript omic data. . CC-BY 4.0 International licenseIt is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. (which was not certified by peer review) The copyright holder for this preprint this version posted April 25, 2025. ; https://doi.org/10.1101/2025.04.24.25326332doi: medRxiv preprint 9 Metho ds Stud y R ec r uitme nt W omen and their p a r tner s wer e r e cr uit ed fr om the Br ist ol C en tr e f or R eprodu c t ive Medic ine (BCR M), wit h r ecruit men t occurr in g in two phase s due t o t he C O VID- 19 p andemic. The fir st phase of r ec r uitment a t BCR M r an from 3 rd Sept embe r 2019 t o 27 th Mar ch 2020. The stud y r eopen ed on 1 April 2021 and c onclu ded on 30 th June 2023. T o be el igible t o t ak e part in t he s tu dy , par ticip a n t s had t o be a g ed 18 o r ov er and t o be under g o in g or planning t o und e r go IVF or ICSI t r ea tment . T r ea tmen t cou ld in volv e use of own or dono r g amet es ( eg gs or sper m) . Wher e t rea tm en t was undert ak en wit h a p a r tn er , both the woma n and th eir part ne r wer e requir ed to enr o ll t oget her t o pr ov ide con s en t f or ac c es s t o BCRM medical no t es . Only pa tie n ts who had c onsen t ed t o r es ear ch c on t act via th e Hu man Fer tiliza t ion and Embry ology Aut hority (HFE A) c onsen t t o disclo s ur e f orm wer e eligible f or pa rt ic ipation ( cd-f orm-v10-16-oct ober-2019.pd f (hf ea.gov .uk ) ) . P art ic ipan t s wer e r ecruit ed prior t o e gg c ollection or fr o z en embry o tr an sf er as part of AR T t r eat ment or f ol l owi ng a sc an a t 7 w e eks g est at i on c onf i rmi ng a vi abl e pr egnanc y . B ef or e t he C OVID- 19 pandemic, r ecruit men t f ollowed a f ace-t o-f ace model, wher e pat ients r eceiv ed an invit ation let t er and par t ic ipant inf or mat ion s heet by post bef or e their clinic v isit and pr ior t o c ommencing dr ug pr ot oco ls. A r e s ear ch midwif e or nur s e obt ained wr it t en inf o rme d c ons ent in per son. P os t-pandemic, due t o r educed in -per son c linic v i sits , t he r ec r uitment pr oc es s shift ed t o r emot e c onsen t. P a tients wer e con t act ed v ia email and t eleph one t o d iscuss stud y part ic ipa t io n, and up t o two f ollow-up c alls wer e made by the r es ear ch t eam. P ar tic ipan t s c ompl et ed onl i ne c onsen t f or ms i n con sul t a t io n wit h a r esear ch mi dwi f e or nu r se. A s par t of . CC-BY 4.0 International licenseIt is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. (which was not certified by peer review) The copyright holder for this preprint this version posted April 25, 2025. ; https://doi.org/10.1101/2025.04.24.25326332doi: medRxiv preprint 10 enr ollme n t, pa r ticipants als o pr ovide d c onsent f or N HS medic al r ec or d link age t o f a cilita te da t a c ollec t ion on pr egnancy and birth o ut c omes . Quest ionnair e P art ic ipan t s c omplet ed a bas eline ques tionnair e on pape r (pr e -C ov id) or o n line (post -Covid ) a t th e poin t o f recruitment. The ques tionnair e a sk ed about employmen t, e duc a tion, cur r ent use of medication s , own and f amily his t ory of c ar diov as cular dis ease , smokin g , alcoho l us e and phy s ic al ac t iv it y . A c opy of the quest ionnair e is pr ovided in t he Supple ment a r y mat e rial. Baseline st udy m easur em en ts Between Sept ember 2019 a n d Mar ch 2020 , s tud y measur es wer e c omplet ed a t the p oint of r ecruit ment. F ollowing t he Apr il 20 21 r eopening , an appoin tment wa s made during the c ons en t call f or the r esear ch t eam t o collect s tudy measur e s. A t th es e vis it s , par tic ipan t s had heigh t, weigh t and bloo d pr es sur e measur ed and blood and u rine s amples t ak en . Saliv a samples wer e sough t wher e part ic ip an t s wer e not able t o or d id no t wan t t o give a bloo d sample. Wher e it was not p o ssible t o con duc t a s tud y vi s it, heigh t , weigh t and blood pr essur e wer e obt ained fr om BCRM n ot es or self-r eported by t he participant. Dat a c o l lect i o n fr om BC RM medical not es F or each f e male und er going tr eatmen t, inf o rma tion on r epr oductiv e his t o r y inc luding pr evious pr egnancie s and birt hs, p r evious f ertilit y tr e a tme n ts, dur a t ion and c aus e s of inf ertility , antr al f ollicle c ount and ant i mulle rian hormone (AMH) lev el wa s pr ovided by the part ic ipan t s and/o r ext r act ed f r o m their BC RM med ic al not es by t he r es ear c h midwiv es . Inf or ma tion o n IVF or ICSI t r eatmen ts was e xtr ac t ed fr o m BCR M medic al r e c or ds , cov ering both ongoing and prior tr eatmen t c y cles (f or those r ec r uit ed aft er t heir 7 - wee k sc an). Th e e xtr act ed dat a in c luded tr e a tmen t type, dr ug r egimens, tr ea t me n t pr otoc ols, f ollic ulogr ams, . CC-BY 4.0 International licenseIt is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. (which was not certified by peer review) The copyright holder for this preprint this version posted April 25, 2025. ; https://doi.org/10.1101/2025.04.24.25326332doi: medRxiv preprint 11 number of eg g s c ollect ed, sperm par amet er s , embry ologic al da t a and tr e a tment out co mes (pr eg n ancy and bir th). T r ea t me n t infor ma tion was co llect ed f or part ic ipant s u p un til the 31 st Mar c h 2024. Collection of inf orma tion on birt h ou t c omes f r om these tr ea t m en ts is st ill ongoing. Pr egn ancy f o l lo w u p If participan ts became pr egnan t f ollowing AR T and h a d a v iable pr egnanc y c onfir med by ultr as ound a t 7 week s g est a tion, t hey and t heir par tner s we r e in v it ed t o participa t e in th e pr egnanc y f o llow up s tud y . In vit a tio ns wer e initia lly sen t by p ost but tr ansitioned t o email f ollowing the C OVID-19 pandemic . Pr eg n a n t women wer e ask ed t o c omplet e an ear ly pr egnanc y ques tionnair e (be f o r e 18 weeks g e st a tion), a lat e p r egnanc y q uest ionnair e (aft e r 28 week s g es t a tion) and a bir th ques tionnair e ( a r ound 2 week s aft er the b i r th of t he ir baby). P art ner s wer e sen t one q uest io nnaire dur ing their partne r ’ s pr egnanc y a nd a ques tionnair e aft er t h e bir th o f th e ir baby . Quest io nnair es c ollec t ed in f orma tion on p r egnanc y health, subst a n c e u s e, f eelings and emotion s, pla n s and e xpect a tions f o r lab o ur a n d par e n thood and birt h e x pe r iences . All pr e gna nc y and birt h quest ionna ir es ar e a vailab le in supplement a r y ma t e r ial. Pr egnan t women and their p a r tner s wer e als o in vit ed t o at t end a clinic at the U niv er sity of Br is t ol du ring pr egnancy . A t th es e clinics, participan ts had a n thr opometry and blood pr essur e mea sur ed a n d under t ook ph y sic al c apability and c ognitiv e t es ts. P art ic ipan t s wer e a s k e d t o c o m pl e t e a n o nl i n e d i e t d i ary f o r 5 d a y s f o l l o w i n g t h e c l i n i c a n d t o w e a r an a ct i v i t y monit or . At the clinic, participan ts ha d blood samples t ak en and pr egnan t women pr o v ided a urine s amp le. Sa liv a samples wer e r eq uest ed wher e it wa s no t pos sible t o pr ovide a b lood sample. Due t o C OVID-19 r estr ic t io ns, v ir tual vis its wer e imp leme n t ed f or a subset of . CC-BY 4.0 International licenseIt is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. (which was not certified by peer review) The copyright holder for this preprint this version posted April 25, 2025. ; https://doi.org/10.1101/2025.04.24.25326332doi: medRxiv preprint 12 part ic ipan t s , during which an equip men t pack wa s p r ovided which enabled collection of a subs et of the measur es fr om t he f ac e- t o -f ace clinic . A t birth, cor d blood and plac en t a s amples, and measur emen ts of th e ba bies wer e co llect ed wher e women d eliv er ed at pa r tic ipat ing hospit als (Univer s ity Hospit als Bris t ol and Wes t on NHS F ounda tion T rus t (St Michael’ s), North Brist ol N HS T rust ( S outhmead Hos pit al) a nd Roy al Unit ed Hos pit a ls B at h NHS Fou nda tion T ru st. W her e pos s ible, v ir tual pos t birth visits wer e c onduct ed 7- 15 da y s aft er the birth of the baby t o c ollect ant hr opome tric inf o rma tion f or both t he m other and baby . All pr egnancy clinics , birth sample c ollec t ions and pos t bir th f ollow-up visit s wer e car ried out by A von Longitudinal Study of Par ents and Childr en (ALSP AC) fieldwork er s us ing identic a l pr ot oc ols as the ALSP AC second g ener a tion pregnanc y clin ics (2 3). Thi s harmoniz a tion allows f or dir ec t comparis o ns between pr egnan c ie s c onceiv ed v ia AR T and n a tur ally c onceived pr egnancies in f utur e analy se s . Biological s am ples Biologic al samples we r e co llect ed a t thr ee k ey ph as e s: baseline, d uring pr e gnanc y and a t the time of bir th (Figur e 3) . At baseline , no n-f ast ing blood, u rine and saliva samples wer e c ollec t ed f rom particip a n t s . Blood samples wer e c ollect ed in E th ylenediaminet et r aac et ic acid (EDT A) tubes, cen trif ug ed with in 24 hour s t o s epar at e plas ma and whit e blo od cells (a sour ce of DN A) and st ored at -80 o C wit hin 2 hour s of pr ocessing. Pla s ma w as st or ed in200µl and 500µl aliquot s , while whit e bloo d c ells wer e st or ed in 1ml aliquot s . In it ial p r oces sin g f or mos t s amples t ook place a t th e BC R M o r Univer sity of Br ist ol labor at ories a t Southmead Hos pit al. All samples hav e sinc e been t r ans f erred f or long t er m st or ag e at Bris t ol Bio r es our c e Labor a t ories (BB L ) . . CC-BY 4.0 International licenseIt is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. (which was not certified by peer review) The copyright holder for this preprint this version posted April 25, 2025. ; https://doi.org/10.1101/2025.04.24.25326332doi: medRxiv preprint 13 F or an initial set of par ticipants, we also c ollec t ed a blood sample in acid c itr a t e de xtr os e (ACD) t ubes f or the cr ea t ion o f immort a liz ed cell lines . Howev er , t his c ollec t ion was discon t inued in Nov ember 2 021 due t o uncert a in ties r eg ar ding pr oces s ing f easibili t y during the C OVID-19 pandemic. A t th e pr egnanc y clinic , non-f a s ting blo od and ur ine sa mples wer e collect ed fr o m pr egnan t women, while pa rt ner s wer e as k ed t o pr ov ide f asting blo od s amp les w h e r e possible. F or these pr egnan cy clinic samples , cho les t er ol and gluc os e lev els wer e obt ained using P TS panels® on a C ar dioChek® P A, and ha emoglobin le v els mea sur ed u s ing a H emo Cue® Hb 201+ s y s t em, within 90 min u t es of obt aining t he sample. Following th ese t e st s, EDT A plas ma , heparin plas ma, s er um and w h it e blood c ell samples wer e aliquo t ed out aft er ce n tr ifuging and s t or ed a t - 80 o C. Cor d blood EDT A b lo od samples and placen t as wer e collect ed a t particip a ting ho s pit als a t the birt h o f the babies and wer e cou rier ed to t he Univer sit y of Br is t ol f or pr oces sin g and s t or ag e. Be f o r e p la cen t a s wer e pr oc e s s ed p hot ogr aphs and measur em en t s wer e t ak en. Placen t as wer e p r ocessed t o ma xim ise futur e us e by s t or in g small pieces i n a v ariety of wa y s including , snap f r ee z ing , fixing in f ormalin and in R N A la t er s o lu tion. Placen t al membr ane and c or d slice s wer e also st o r ed. Cor d blood was c en t rifug ed and s epar a t ed in to EDT A plasma (aliq uot ed int o 200u l and 500u l) a n d w h it e blood cells, which wer e st or ed a t -80 O C Us e of b iologic al samples A t o t al o f 823 plasma samples wer e c ollect ed, co mprising 372 woman a t baselin e, 307 part ner s a t baseline, 57 woman dur ing pr egnan cy , 28 p artner s dur ing p r e gn a n cy and 59 of f spring co r d bloo d. T o dat e, N MR met abolit e q uantific a tion h a s b ee n c omplet ed on 200 samples t a k en at bas eline in the women and 167 t ak en a t b aseline fr om part ner s. Wit h the . CC-BY 4.0 International licenseIt is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. (which was not certified by peer review) The copyright holder for this preprint this version posted April 25, 2025. ; https://doi.org/10.1101/2025.04.24.25326332doi: medRxiv preprint 14 c ompletion o f bio -sample collectio n ( s ee Figur e 1 f o r final numb er s ) , N MR met abolit e quan tifica tion f o r all baseline sample s fr om wo man and part ner is schedul ed f o r c ompletion in lat e 2025. Thes e da t a will be used t o in v est iga t e r es ear c h ques tions outlined in the Planned Analy ses se ction, with comparis o ns ma d e t o the AL SP AC -G 2 c oho r t, which includes equiv alent NMR met abo lit e a ssessme n ts during pr egnancy and in c or d blood. The pr ot oc ol used in this s tud y f or pr eparing a n d st oring pla cen t al tiss u e i s iden t ical t o that use in ALSP AC-G2 . We ar e curr en tly (Apr il 2025) pr ep a ring all 70 BRIST-IVF placen t a samples and 70 ag e m a t ched ALSP A C-G2 placen t al s amp les f or of tr ans crip t omic and DNA meth yla tion analyse s t o e xplor e diff e r en c es in t he s e biologic al mea sur es bet ween AR T and na tur al c onception s . Wit h fu tur e funding , additiona l analy se s a r e planned, inc lud in g s er um p r ot eomic analy se s on baseline woman and part ner seru m s amples and mas s spe ctr ometry met abolit e pr o f iling of baseline a nd pr egnancy urine sa m ples in women and the ir partner . Dat a manag emen t Dat a have been collected and stor ed using the Res ear c h Ele ctr onic Dat a Captur e (REDCAP) software a nd wor kflow technolog y hosted at the U niversity of Bristol (24) . Res ear c h data i s stored in a s epar ate filestore from p ersonal data and o nly t hree r es ear c her s hav e acce ss to personal identifier s . P a tien t in volvemen t A pa tien t advis or y gr oup of people w ho h a d o r were under going IVF tr ea tmen t was set up in early 2019 t o advise on the r elev ance , ac cept ability of s tudy pr oc e s s e s and document a t ion. E thical appr oval . CC-BY 4.0 International licenseIt is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. (which was not certified by peer review) The copyright holder for this preprint this version posted April 25, 2025. ; https://doi.org/10.1101/2025.04.24.25326332doi: medRxiv preprint 15 E thics a p pr ov al f or t he st ud y wa s obt ained f r om th e U K Na tional Healt h Ser vic e Sou th We s t- Fr encha y R es ear ch E th ic s Commit t ee (IRAS pr o jec t ID 236773, Initial appr o val 10/7/ 201 9) . . CC-BY 4.0 International licenseIt is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. (which was not certified by peer review) The copyright holder for this preprint this version posted April 25, 2025. ; https://doi.org/10.1101/2025.04.24.25326332doi: medRxiv preprint 16 R esults Char ac t erist ic s of t he r ecrui t ed s am ple Figur e 2 shows the f lo w of p articipan ts fr om initial s cr eening thr ough in vit a tion t o re c r u i tm e n t . Prior t o th e C OVID-19 pandemic , 2 1 3 cou ples or sin gle wo men wer e scr eened, while 2,225 wer e scr eened f ollowing r e sumpt ion of AR T s erv ice s. O f these 2,3 38 c oupl es / s in gle women, 1,214 met the s tudy ’ s eligibilit y crit eria, w it h 5 02 (4 1% ) suc c es sf ully r ecr uit ed. The f ina l c ohort co mprises 5 02 women and 4 6 5 p a r tner s (4 45 women with a male p artner (89%), 20 women with a f emale par tne r (4%) and 37 women under going tr ea t ment wit hout a part ner(7%)) . S ince r ecru it me n t one s ingle woman has with dr awn her c ons en t. Char a ct er istics of t he r ec r uit ed wo men and t heir part ner s ar e shown in T ables 1 and 2. Wher e same-se x co uple s wer e unde r g o in g r ecipr oc al IVF /IC SI, f or da t a c ollection p urposes we c onsid er ed t he woman who wo uld c arry t he embr y o as the woma n under going tr eatment. W omen under g o in g t r e atmen t had a mean ag e of 35.8 y ear s (SD = 4.4) a t r ecruit ment and a mean BMI of 25.1 (SD = 4.4). The majority (92%) wer e of whit e et hnicity . A t the time of r ecruit me n t, 251 women (50%) had ne v er been p r egnan t, a n d 374 (75%) had not had a pr ev ious liv e birt h. Their p artner s had a mean ag e of 37.6 y ear s (SD = 5.8) and a mean BMI of 26.8 (SD = 4.3), with 94% ident ifying as whit e. T r ea tm en t cy c les and pr eg n an c y ou t c o mes Amo ng the 501 c ouples and s ingle w omen who wer e s ucce s s fully r e cruit ed and r emained in the s tudy , 12 (2%) wer e enr o lled following a c onfir m ed viable p r egnanc y a t a 7 - wee k gest at ional s c an. . CC-BY 4.0 International licenseIt is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. (which was not certified by peer review) The copyright holder for this preprint this version posted April 25, 2025. ; https://doi.org/10.1101/2025.04.24.25326332doi: medRxiv preprint 17 F or this s tudy , we de f ined a tr ea tmen t cy cle as an eg g co llection with or w it hout a fr es h embr yo tr ansf er or a fr o z en embry o tr ans f er . Be t ween r ec r uitme n t and Mar ch 2024 , 490 women unde r went a t o t al o f 1,05 5 ART c y cle s . F ollow up time in the s tudy f or the 11 wome n who wer e r ecruit ed but did n ot und er go an y r elev an t cy c les r ang ed fr om 1 1-23 mon t hs . Of the 1,0 55 AR T cy cles, 1 25 wer e IVF (in 69 o f these the woman had a fresh embry o t r ans f er), 429 wer e ICSI (in 273 of these the wo man had a fr es h embry o tr ansf er ) an d 501 wer e fr o zen embr yo t r ans f er s (s ee Figur e 3). Of the 843 embry o tr ansf er s , 458 ( 54 %) r esult ed in a p ositiv e p r egnancy t es t, 378 (45%) in a neg a tiv e pr egnancy t est and 7 out c omes wer e unkno wn. O f the 45 8 pr egnancie s, 96 wer e biochemic al or end ed in a m is c arr iage pr ior t o the 7 -week s c an. T r ea tmen t inf or m a tio n r eport ed in t his paper is t ak en fr om a da t a download on 15 Augus t 2024. R epor t ed numbe r s ma y c h a n ge wher e t her e ar e up d at es t o clinic r ec or ds. Pr egn ancy que s tio nnair es and cl in ic F ollowing a pos it ive 7-week sc an c o nfirming a v iable p r egnancy , 305 wo men and , wher e appr o priat e, t heir par tner s wer e in vit ed to par ticipat e in t he p r egnancy f ollow up st udy . O f these 305 , 1 9 had t wo pr egnan cies r e sulting in a t o t al of 324 pr egnancie s o ver the c our se of the st udy (see Figur e 4). In vit at ions wer e not iss u ed f or 38 p r egnancies due to clos ur e of clinics f or t he Covid-19 pandem ic, lack of c onsent f or pr egnancy f ollow up, or the s tudy t eam not be ing not ified o f the pr egnanc y . Amo ng the 324 pr egnan c ies, part ic ipa t io n in pr egnancy f ollow-up wa s a s f ollows: 246 (76%) pr egnancies r eturned a t least one pr egnanc y quest io nnair e, 23 3 pr eg n a n c ie s ha s a c omplet ed bir t h quest io nnair e, 117 wome n (f or 119 pr egnan cies ) and 39 partn e r s . CC-BY 4.0 International licenseIt is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. (which was not certified by peer review) The copyright holder for this preprint this version posted April 25, 2025. ; https://doi.org/10.1101/2025.04.24.25326332doi: medRxiv preprint 18 part ic ipa t ed in pr egnancy c linics a t the Univ er sit y of Bris tol and 70 placen t as and 58 c or d blood samples wer e d onat ed. Pr egn ancy and b i r th ou t come s Da t a collection on p r egnancy out c omes f ollowing tr e a tme n ts is ongoing. We ar e co llecting da t a f or all pr egnancies of st udy par tic ipan t s fr om BCR M me dical no t es. This inf o rma tion includes pr egnancy out c ome (miscar riag e, st illbir th, ter min a tion of pr egna ncy , liv ebirt h ), se x and birt h we ight of the baby . Additionally , pr egnancy and b ir th ou t c omes ar e being r etriev ed fr o m o bst et ric r ec or ds f or women who ha v e c onsen t ed t o ac ces s t o their NHS medical r ecor ds and ha ve giv en bir th at loc al participa t ing hospit als. The s e NHS r ecor ds inc lude a nt ena t a l measures (height, weight, blood pr es sur e, haemoglo bin), ho spit al admis sion s, diagno s es during p r eg nancy ( ges t a tional hypert ens ion, pr e- ec lampsia , g e s t a tional diabet es), det ails of labour (how labour st art ed, mode of d eliv e r y ) , child measur emen ts , APGAR s c or e and an y con g enit al ano malies diagnosed a t bir th. W e an ticipa t e that we will have NHS c linic al dat a f or a ppr o xima tely 200 pr egnancies , a nd tha t these da t a will be av ailable f o r r es ear ch use b y the end of 2025. . CC-BY 4.0 International licenseIt is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. (which was not certified by peer review) The copyright holder for this preprint this version posted April 25, 2025. ; https://doi.org/10.1101/2025.04.24.25326332doi: medRxiv preprint 19 D i sc u s si o n Str ength s and Limit a tion s BRIST-IVF i s a la r g e c linic al c ohor t of women and t h eir partner s under going ART , with e xt ensiv e d e mogr aphic, lif es tyle, biologic a l s amp les and tr ea t m en t-r e la t ed inf or mation a v a ilab le. W e h av e als o be en a ble t o f ollow u p the pr egnancies and bir ths via ques tionnair es, c linic s and medical r ec or d link age. Impor t antly , pr egnancy da t a collection f ollowed the same pr ot oc ols as the ALSP AC G2 (2 3), allowing f or d ir ec t comparisons between A R T p r egnancies and na tur a lly c onceived p r egnancies . Our s tudy c ohort c losely mirr o r s t he U K popula tion unde r going IVF tr ea t m en t, bo t h in t er ms of a v er ag e ag e and f a mily t ype. In 2 0 21, t he a ver age a g e o f I VF pa tien t s in the UK wa s 36.0 y ear s , and 89% of women under going IVF had a male part n er , 4% had a f e male partner , an d 6% had no part ner(25) . The BRIST-IVF c ohort ha s a higher pr oportion of pa r t icipant s of whit e ethnicity (92 % ) compar ed t o the UK a ver ag e f or IVF t r eatme n t in 2 020-2 1 (77%)(26). Ho wev er , t his pr oport ion aligns wit h the et hnic compos it ion of t he South W e st r egion (9 3% of r esidents ar e o f whit e e t hnicit y)(27). Despit e these s tr eng t h s , limit a tion s in clude t he modes t r espon s e, with o nly 41 % of women/ cou ples scr eened agr eeing t o p articipa t e. One imp ort ant r eas on f or the limit ed r esear ch part ic ipa t io n wa s t he high p r op ortion of patient s ( 38% ) who did not co ns en t t o be c on t act ed f or r esear ch when c ompleting t he HFE A f orm prior t o beginning tr eatme n t a t the BCRM. This pr oport io n is s imilar t o findings fr om a s tudy c onduct ed a t a f ertility c en t r e in nor thern England bet ween 2010 and 2019, wher e 47% of pa tien t s de clined t o pr ovid e c ons ent f or c ont act r es ear ch (28) . . CC-BY 4.0 International licenseIt is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. (which was not certified by peer review) The copyright holder for this preprint this version posted April 25, 2025. ; https://doi.org/10.1101/2025.04.24.25326332doi: medRxiv preprint 20 Whils t we ha ve near c omp let e dat a fr om baselin e qu es tionnair es and t r e a tmen t d a t a f r o m BCRM medic al r ec or ds, only 76% of women in v it ed c on tribu t ed t o t he pregnanc y f ollow up s tudy b y answering a quest ionnair e or a tt ending c linic. Howev e r , obst etric da t a will be a v a ilable f or all wom en who consen t ed t o NHS medical r ec or d abst r action and deliv ered within loc al partic ipa ting h os pit a ls. Add it ionally , bloo d and urine samples wer e f or the mos t part t ak en aft er women had st art ed tr ea tment pr o t oc ols meaning th at biologic al meas ur es e.g. met abolit es may be influenced by dru g s t ak en a s par t of t he I VF p r oc es s. However , det ailed inf or mation on t rea tm en t and drug pr o t oc ols ha s been collec t ed, allowing f or appr o priat e adjus tment in analy ses . Planned a n alyses The BRIST-IV F s tudy will lev er age its ext ens iv e dat aset t o inv es tigat e s ev er al k ey r es ear c h ques tions r ela t ed t o ART , met abo lic pr ofiling , and perina t al ou t come s. Spec ifically , we aim t o addr ess the f ollowing: 1. D o d i f f e r e n t A R T p r o t o c o l s i n f l u e n c e m e t a bo l i c p r o f i l e s ? T r e a t m e n t c o m p a r i s o n s t o include: a. Us e of IVF v s ICSI b. Transfer o f frozen vs Fresh embr yo 2. Do metabo lic pr ofiles in fluence liv e birth s ucce ss and perinatal ou tcome s (small for ges t ational age, la r ge for gestational age, pret erm bir t h)? 3. Do metabolic pr ofiles influence pregnanc y complications (mis carriage, h y per tensiv e disorder s of pr egnancy, ge stational diabetes)? . CC-BY 4.0 International licenseIt is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. (which was not certified by peer review) The copyright holder for this preprint this version posted April 25, 2025. ; https://doi.org/10.1101/2025.04.24.25326332doi: medRxiv preprint 21 4. Dep e n ding on the results fr om 1 to 3 a bov e we will explor e whe th e r v ar iat ion in metabo lic pr ofiles in th os e u ndergoing ART mediate as soc iat ions between diff erent ART appr oaches and pr egnan cy or p erinatal ou tcomes 5. Do metabo lic profiles differ be twe en p regnancies con ceived by ART and t hos e conceiv ed nat urally? 6. Ar e the re differences in plac ent al t is sue DNA met hyla t ion and gene express ion between AR T a n d natura lly conceived pr egnanc ies ? 7. Ar e t here differen c es in placental pa t hology bet ween bet w een AR T an d natur ally conceiv ed pr egnan c ies? T o addr ess que st ions 5 t o 7, we will use da t a f r om the ALSP AC G 2 c ohort (23) as a c omparison gr o up of n at ur ally c once i v ed pr egnancies. Given tha t B RIST-IVF par ticip a n t s wer e f o llowed up us ing id en tic a l clinic p r ot oc ols, labor at ory pr ocedur es, and pla cen t a l tiss u e analy se s , this w ill allow f o r direct and meaningful c ompar is ons bet ween A R T and non-AR T pr egnancies .

The BRIST-IVF s tudy , with its c ompr ehens iv e me t abolic, clinical, and b iologic al dat a, is uniquely positioned t o addr ess s ome o f the lim it a t ions of birt h c ohor t and elec t r onic health r ec or d s tudies in AR T r esear ch. B y in t egr at ing det ailed t rea tmen t inf o rma tion with biospecimen analy s i s and pr egnancy f ollow-up, t he B RIST-IVF st udy off er s a valuable r esour ce f or adv an c ing our under s t anding of the pr e d ict o r s a n d co n sequences of AR T c onception, wit h impor t an t implic a tions f or optimizing f e r tility trea t m ents and impr ov ing AR T pr egnanc y out comes . . CC-BY 4.0 International licenseIt is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. (which was not certified by peer review) The copyright holder for this preprint this version posted April 25, 2025. ; https://doi.org/10.1101/2025.04.24.25326332doi: medRxiv preprint 22 De c lar a t ions E thics appr oval and con sen t t o participa t e All part ic ipant s pr ovided inf ormed c o nsent t o part icipa t e in the s tudy . E thic s appr ov al f or the s tudy wa s obt ain ed fr o m the UK N a tion al Healt h Service Sout h W est - Fr enc h ay R e s ear c h E thics Commit t e e ( IRAS pr oject ID 236773, Initial appr ov al 10/ 7/20 19) . Consen t f or pub li ca tio n Not applicable A vailabil ity of da t a and ma t e rials We ar e k een t o work w it h c ollabor a tor s and welco me enquir ie s f r om resea r c h er s who ar e in t e r est ed in the m easur es we ha ve c ollect ed and h av e ideas f or r es ear c h q uest io ns be y ond those list ed abov e. In fut ure, r esear cher s will be able t o app ly t o use anon ymised s u b s et s of the da t a f or appr ov ed r es ear ch ques tions. Comp eting in t er est s The author s declar e that they ha v e no competing inter es ts. Funding This s tud y was f unded by the Eur op e an R esear ch Council under t h e Eur opean Union’ s Ho rizon 2020 r esear ch and innov a tion pr ogr am ( gr an t agr eemen ts No 10 102156 6) and the NIHR Biomedic a l R es ear c h Cen tr e a t Univ er sity Hos pit als Br ist ol N HS F ounda tion T rus t and the Unive r s ity of Bris t ol. The W ellc o me T rus t and UK Medic al R esear ch Co uncil fund the ALSP AC G 2 co hort and the c lin ic f ac ilities that we r e us ed in participan t f ollow-up (02215/2 / 13 /2). A. T a n d D .A.L ar e suppor t ed by t he UK Medic al R esear ch C ouncil . CC-BY 4.0 International licenseIt is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. (which was not certified by peer review) The copyright holder for this preprint this version posted April 25, 2025. ; https://doi.org/10.1101/2025.04.24.25326332doi: medRxiv preprint 23 (MC_U U_00032 /05). The views e xpr e s sed in this public a tion ar e those of the auth o r(s) and not n ec es s ar ily t hos e of an y of the funder s , t he N HS, o r the Departmen t of Healt h and Socia l Car e. None of t he funder s influenced the s tudy desi g n or analyse s. Au thor s ’ con tr ibutio ns DAL , V A, A T , SN, KAR designed the s t udy . A T , TK and J P manag ed da t a c ollection. SP and AG desig ned a nd manag ed the biologic a l sample pr oces s ing . A J and PW pr ovided s cientific input dur in g da t a co llection. A T a n a ly sed t he da t a and A T and DAL dr aft ed the ma n us cript . All autho r s c ommen t ed on a dr aft o f the manus cript. Acknowledg em en ts We a r e v ery gr a t e ful t o the f ollowin g people f or their help w it h d a t a c olle c t ion: the t eam of r esear ch midwiv es and nu r s es (Alis on Kirby , Naomi Mallinson, Annie Deacon, Mic h elle Magg s, Ashleigh Pr omnitz , S o phie W ic kham, Vict or ia Car ey) a t N or th B rist ol NHS T rus t who r ecruit ed par ticipan ts f or this s tud y , t he s t af f a t the Bris t ol Cen tr e f or Reprodu c t ive Medic ine who f a cilit a t ed th is r esear ch , t he r esear ch midwiv es a t N o rth Bris t ol NHS T r ust , Univer s it y Ho s pit als Bris t ol NHS F ounda t io n T r us t and The R o y al U nit ed Hospit als B at h NHS F ounda tion T rus t who collect ed bir th samples and t he Children of t he 90s fieldwork ers who c ondu ct ed the pregnanc y c linic s . We would a lso lik e t o thank the member s of t he p at ien t adv isory gr oup who pr ovided valuable adv ice a n d f eedbac k on s tudy pr o c es ses and documen t a tion. . CC-BY 4.0 International licenseIt is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. (which was not certified by peer review) The copyright holder for this preprint this version posted April 25, 2025. ; https://doi.org/10.1101/2025.04.24.25326332doi: medRxiv preprint 24 . CC-BY 4.0 International licenseIt is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. (which was not certified by peer review) The copyright holder for this preprint this version posted April 25, 2025. ; https://doi.org/10.1101/2025.04.24.25326332doi: medRxiv preprint Figure 1. Biological samples collected for BRIST-IVF Icon made by Freepik/DinosoftLabs from www.flaticon.com . CC-BY 4.0 International licenseIt is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. (which was not certified by peer review) The copyright holder for this preprint this version posted April 25, 2025. ; https://doi.org/10.1101/2025.04.24.25326332doi: medRxiv preprint Figure 2. Flowchart of study recruitment 26 . CC-BY 4.0 International licenseIt is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. (which was not certified by peer review) The copyright holder for this preprint this version posted April 25, 2025. ; https://doi.org/10.1101/2025.04.24.25326332doi: medRxiv preprint Figure 4. Overview of treatment cycles in IVF/ICSI Ovarian s tim ula t ion ai m s to s t i m ula te f o lli cles t o pr oduce mul t ip le ma ture oocy tes to be r etrie ved v ia fol lic u l ar aspiration. Matu re oo c y tes ar e fer tilis ed in vitr o via IVF or I CSI and su c c es sful fert il isation r es ult s in t he f orm ation of embry os. Embry o s of sufficient quality can be tr a n s fer red into th e u terus wit hout freezing (f resh embr y o tr ansfer) or may be frozen and stored for fut u re use. Frozen embry os can then be t haw ed and tr ansfer red into th e . CC-BY 4.0 International licenseIt is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. (which was not certified by peer review) The copyright holder for this preprint this version posted April 25, 2025. ; https://doi.org/10.1101/2025.04.24.25326332doi: medRxiv preprint 28 uter us (fr oz en embr y o tr ans f e r ). F or each o v ar ia n stimulation c y cle, t here may be b oth a fr es h embryo t ransfer and multiple f rozen embr y o transfer s . C ouple s /individuals ma y under go multip le ovarian stimulation c ycle s . Icon made by Freepik/cube29/Ba c k wood s /Sma shicons from www.flaticon. com . . CC-BY 4.0 International licenseIt is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. (which was not certified by peer review) The copyright holder for this preprint this version posted April 25, 2025. ; https://doi.org/10.1101/2025.04.24.25326332doi: medRxiv preprint Figure 4. Flowchart of the pregnancy follow up to date 29 . CC-BY 4.0 International licenseIt is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. (which was not certified by peer review) The copyright holder for this preprint this version posted April 25, 2025. ; https://doi.org/10.1101/2025.04.24.25326332doi: medRxiv preprint 30 T able 1. Char act er ist ic s of th e wo me n r ecruit ed t o t he st udy ( N=501)* Mea n ( SD ) /N (% )/ M e d i a n (IQ R) Ag e Y ea r s 35.8 (4. 4) E thnicity Whit e Other et hnic it y 461 (92 % ) 38 (8%) BM I Kg /m 2 (N=477) 25.1 (4. 4) Smoking Never s mo k er Fo r m e r s m oke r 371 (74 % ) 129 (26 % ) Alc ohol use be f o r e tr yi n g t o conceiv e Never Mon t hly or les s 2-4 times a mon th >once a week 46 (9%) 135 (27 % ) 156 (3 1% ) 161 (32 % ) Number of t imes per w eek doing moder a te t o vigor ous ex e r c i s e 0 1-2 3-4 5+ 100 (20 % ) 249 (5 0% ) 122 (24 % ) 29 (6%) Has a degr ee Ye s No 376 (7 5% ) 125 (25 % ) Pr evious pr egnan cies 0 1 2 3 or mo r e 251 (5 0% ) 135 (27 % ) 54 (11%) 61 (12%) . CC-BY 4.0 International licenseIt is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. (which was not certified by peer review) The copyright holder for this preprint this version posted April 25, 2025. ; https://doi.org/10.1101/2025.04.24.25326332doi: medRxiv preprint 31 Pr evious live births 0 1 or mo r e 374 (7 5% ) 127 (25 % ) An ti M ulle rian Ho rmon e (A M H ) pmol/L ( N =479) 14.1 (7. 1 , 26 .0) *1/502 w om a n withd r ew f r o m the s tud y . Mis s ing da t a f or eth n ic it y (N =2), BM I (N=24), Smoking ( N=1), Alc ohol (N= 3 ), Ex er c i s e (N =2) , AMH (N =22 ) T able 2. Char act er ist ic s of th e partn e r s ( N =465) M ea n ( SD ) / N(% )/ M ed i an (IQR) Ag e Y ea r s 37.6 (5.8) Se x Male Fe m a l e 445 (96%) 20 (4%) E thnicity Whit e Other et hnic it y 433 (94%) 30 (6%) BM I Kg /m 2 (N=422) 26.8 (4.3) Smoking Never Fo r m e r C u rre nt 293 (63%) 164 (35%) 7 (2%) Alc ohol use be f o r e tr yi n g t o conceiv e Never Mon t hly or les s 2-4 times a mon th >once a week 44 (10%) 91 (20%) 145 (32%) 181 (39%) . CC-BY 4.0 International licenseIt is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. (which was not certified by peer review) The copyright holder for this preprint this version posted April 25, 2025. ; https://doi.org/10.1101/2025.04.24.25326332doi: medRxiv preprint 32 Number of t imes per w eek doing moder a te t o vigor ous ex e r c i s e 0 1-2 3-4 5+ 68 (15%) 158 (34%) 135 (29%) 102 (22%) Has a degr ee Ye s No 284 (61%) 180 (39%) Mis s ing da t a f o r ethnicity (N=2), BM I ( N =43), Smo king ( N=1), Alc ohol (N=4), Ex er ci s e (N=2) . CC-BY 4.0 International licenseIt is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. (which was not certified by peer review) The copyright holder for this preprint this version posted April 25, 2025. ; https://doi.org/10.1101/2025.04.24.25326332doi: medRxiv preprint 33 Box 1. The impact of the Covid-19 pandemic on the BRIST-IVF Study BRIS T-I VF wa s s u spend ed f o r 13 mon th s betw e e n Mar ch 2020 an d A pr il 2021 due t o Cov i d 19 r e str ic tio ns and s t a ff redeploy me n t . A s a r e sult , the st u dy wa s unabl e t o me et it s the initial r ec r uit me nt t a r ge t and wa s clo sed e arli er than pl ann ed a s pa rt of the NIHR R e sea r c h and R ec ove ry Re s e t P ro g ra m m e . Pla nne d da t a col lection w a s aff ec t ed i n t he f ollowing wa y s : • During th e clos ure p eriod i t was no t po s sibl e to t r a ck t h e tr e atmen t outcom e s of a ll rec r ui ted par ticipa n ts in r ea l time a nd in vi t e particip a nt s t o preg na ncy follow up. Howe ver, we have s till b een a bl e to c olle c t informa tion on p r eg nanc y outco m e s fr om BCRM me dica l not es a nd fr om ob ste tr ic note s wher e par t i cipan ts h av e co n s ent e d t o ac ces s t o N HS medic al r ec or d s . • Redu ction in t he numbe r of fac e - to -fac e consul tati on s at t he B CRM during and a ft er the pan demic mean t tha t the c on s ent pr oce s s moved f rom fa c e- to -f ace t o t e le phone app ointme nt s a nd that s t ud y mea s ure s v isit s we re no longe r c arr i e d out on the d a te o f rec r uit m e nt. Thi s me ant th at : - Bi ologic al sample s w er e mo st l y c ollec te d af ter tr e a t m en t pro t oc ol s had b e en in itia te d. - F ewer stu dy me asure s vis it s were com pleted , and mo re study mea sure s w ere se lf - repor ted o r info rmati on was col l ected f r om fe rtili t y cen tre no te s . • Pregn a ncy clini cs move d from face - t o- fa c e to vir tual visi t s for seve ra l month s a ft er t h e stu dy reope ne d mean i ng that a mor e lim ited se t of me asure s we re coll ect ed f or s ome partic ipan t s. • Coll ec tion o f blood sample s for the purp os e of ma king immortal c ell li ne s wa s sto ppe d a s it w as uncle a r wheth er th e se c ould be p r oc es sed f o ll owin g Covi d. . CC-BY 4.0 International licenseIt is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. 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