{"paper_id":"8763c0c1-202a-4efb-b75e-2f7c4333b085","body_text":"Over the past decade, researchers have investigated factors that may influence women’s fertility. One of the possible causes of reduced fertility is uterine fibroids, also known as leiomyomas ( Khaund and Lumsden, 2008 ). Fibroids are benign tumours of the myometrium, which develop in or around the uterus. The prevalence of fibroids ranges from 4.5% to 68.6%, partly depending on main risk factors such as ethnicity and age ( Stewart  etal. , 2017 ). The incidence is significantly higher in women of African origin; these women may display fibroids at a younger age and may suffer from more severe symptoms ( Baird  etal. , 2003 ). Oestrogen and progesterone stimulate the growth of uterine fibroids and, as a consequence, development usually starts after menarche. During the reproductive years fibroids are most progressive and usually regress after menopause ( Moravek and Bulun, 2015 ). Studies indicate that at least 50–75% of fibroids are asymptomatic ( Divakar, 2008 ;  Stewart  etal. , 2016 ). However, if they become symptomatic, the most common symptoms are extensive blood loss during menstruation, pelvic pain, sexual dysfunction, and pressure complaints frequently leading to urological and/or gastrointestinal symptoms ( Stewart  etal. , 2016 ;  Bochenska  etal. , 2021 ). Fibroid-related symptoms are associated with the size, number, and locationof the fibroids. The location of fibroids are classified according to the International Federation of Gynecology and Obstetrics (FIGO) system ( Munro  etal. , 2018 ). Generally, fibroids do not influence women’s fertility, however, in women with infertility, fibroids are more prevalent. Fibroids are diagnosed in approximately 5–10% of women with infertility, and are estimated to be solely responsible for infertility in 2–3% of patients ( Khaund and Lumsden, 2008 ;  Klatsky  etal. , 2008 ;  Brady  etal. , 2013 ;  Bosteels  etal. , 2018 ). Next to infertility, research has shown that fibroids are associated with early pregnancy loss ( Benecke  etal. , 2005 ;  Somigliana  etal. , 2007 ;  Khaund and Lumsden, 2008 ;  Klatsky  etal. , 2008 ;  Pritts  etal. , 2009 ) and adverse obstetric outcomes like abdominalpain during pregnancy (because of excessive fibroidgrowth or fibroid necrosis), preterm labour, placenta abnormalities, intrauterine growth restriction, and increased rates of caesarean section and postpartum haemorrhage ( Exacoustos and Rosati, 1993 ;  Coronado  etal. , 2000 ;  Benson  etal. , 2001 ;  Sheiner  etal. , 2004 ;  Ouyang  etal. , 2006 ;  Qidwai  etal. , 2006 ;  Cook  etal. , 2010 ). The extent of the impact on reproductive outcomes appears to largely depend on the location of the fibroid. For example, it is suggested that fibroids distorting the uterine cavity could mechanically alter implantation, where intramural fibroids have more impact on functionality of the myometrium, as both negatively influencing receptivity ( Casini  etal. , 2006 ;  Brady  etal. , 2013 ;  Christopoulos  etal. , 2017 ;  Behbehani  etal. , 2018 ). Up to now, the most intriguing questions are about how uterine fibroids influence fertility and pregnancy, and which underlying pathophysiological mechanisms have been studied sufficiently. Constructed on the available literature, we postulate seven hypotheses to clarify the association of uterine fibroids with infertility and unfavourable pregnancy outcomes. In addition, we determine the knowledge gaps as perspectives for future research.  Figure 1  and  Table 1  show an overview of the postulated hypotheses on the underlying mechanisms that may be responsible for the detrimental effect of fibroids on fertility.\nAn overview of the seven postulated hypotheses responsible for the detrimental effect of fibroids on fertility .\nSeven postulated hypotheses on the underlying mechanisms that could be responsible for the detrimental effect of fibroids on fertility.\n\nSexual dysfunction in premenopausal women is a relatively common disorder with a prevalence of 30–50%, and fibroids may be associated with sexual dysfunction; however, prospective data exploring this association are limited and conflicting ( Moshesh  etal. , 2014 ;  Chang  etal. , 2019 ). Fibroids can cause pelvic pain and/or dysmenorrhea resulting in dyspareunia ( Ferrero  etal. , 2006 ;  Ertunc  etal. , 2009 ). Additionally, fibroids can cause heavy and/or prolonged menstrual bleeding; this combined with dyspareunia could interfere sexual arousal, and as a consequence negatively influence the frequency of intercourse, resulting in a reduced probability of pregnancy ( Moshesh  etal. , 2014 ). Several studies have shown that the presence of fibroids had a significant association with deep dyspareunia and impaired sexual satisfaction compared to the absence of fibroids ( Lippman  etal. , 2003 ;  Ertunc  etal. , 2009 ;  Moshesh  etal. , 2014 ). No association was found between fibroid size and pelvic floor problems, although fundal fibroids were more associated with dyspareunia ( Moshesh  etal. , 2014 ;  Bochenska  etal. , 2021 ). Treatment of fibroids, like myomectomy, may relieve pelvic pain during intercourse, thereby improving sexual function in these women ( Ertunc  etal. , 2009 ;  Don  etal. , 2022 ,  2023b ;  Dedden  etal. , 2023 ). However,  Ferrero  etal.  (2006)  did not find an association of fibroids with impaired sexual function, neither did they find an association between fibroid location and dyspareunia ( Ferrero  etal. , 2006 ). In conclusion, sexual dysfunction in an infertile couple is a complex issue and, along with possible treatment of fibroids, it might be necessary to also address other medical or psychosocial problems ( Berger  etal. , 2016 ).\n\nFibroids can obstruct sperm cells from reaching the oocyte, obstructan embryo from reaching the uterine cavity and/or hinderimplantation, by compressing the endometrium through mechanical distortion of the endometrial cavity, the interstitial part of the fallopian tubes or the cervix ( Oliveira  etal. , 2004 ;  Horne and Critchley, 2007 ;  Donnez and Dolmans, 2020 ). Extrinsic compression by large subserosal or intramural fibroids can cause gross anatomical distortion of the interstitial part of the fallopian tubes, resulting in tubal factor infertility, as this could hinder sperm or oocyte penetration, and consequently conception ( Hart, 2016 ;  Zepiridis  etal. , 2016 ). It is essential to differentiate between fibroids that distort the uterine cavity, submucosal fibroids FIGO type 0,1 and 2 and intramural fibroids FIGO type 2-5, and fibroid types that do not distort the uterine cavity, intramural fibroids FIGO type 3,4,5 and subserosal fibroids FIGO type 6,7 ( Munro  etal. , 2018 ). The negative effect of fibroids distorting the uterine cavity on fertility have been widely documented, as women with cavity distorting fibroids are demonstrated to have significant lower implantation, clinical pregnancy and live birth rates, and significantly higher miscarriage rates ( Pritts  etal. , 2009 ;  Donnez and Dolmans, 2016 ). Myomectomy of submucosal fibroids appears to restore reproductive potential ( Klatsky  etal. , 2008 ;  Pritts  etal. , 2009 ;  Bosteels  etal. , 2018 ). While there is still debate about whether myomectomy of intramural and subserosal fibroids can improve reproductive outcomes, if the myomectomy releases the mechanical compression caused by large fibroids, this could in theory improve oocyte and sperm transport, and thus fertility. Additionally, when patients simultaneously suffer from fibroid-related symptoms, myomectomy of submucosal, intramural, or subserosal fibroids would not delay the time to live birth despite the required recovery time after surgery and the risk of intrauterine adhesions ( Don  etal. , 2022 ). On the other hand, if a fibroid is located close to the interstitial part of the fallopian tube, it may be challenging to restore its anatomy after myomectomy without disturbing the transport functions. Futures studies are needed to elucidate the benefit of myomectomy in these specific cases.\n\nThe third hypothesis states that fibroids possibly disturb the frequency, direction, amplitude, and coordination of junctional zone peristalsis resulting in inadequate oocyte and sperm transport. The contractility of the subendometrial layer or inner myometrium in a non-pregnant uterus moves in specific patterns and directions depending on the phase of the menstrual cycle, also known as junctional zone peristalsis ( Harmsen  etal. , 2022 ). By junctional zone peristalsis, the journey of sperm through the genital tract to the fallopian tubes is enhanced during the late follicular phase until ovulation, and after ovulation, junctional zone peristalsis decreases dramatically to support implantation ( Bulletti and de Ziegler, 2006 ;  Fanchin and Ayoubi, 2009 ;  Huang  etal. , 2022 ). Altered junctional zone peristalsis is considered to cause IVF failure, because alterations to the direction and/or frequency of the contractions could induce the evacuation of an embryo out of the uterine cavity ( Bulletti and de Ziegler, 2006 ;  Fanchin and Ayoubi, 2009 ). Additionally, the contraction amplitude is of influence, as it has been suggested that frequent contractions of low amplitude can favour implantation ( Lyons  etal. , 1991 ). Fibroids are reported to negatively influence the frequency, direction, and coordination of junctional zone peristalsis, although their influence on the amplitude of contractions has not been explored ( Nishino  etal. , 2005 ;  Yoshino  etal. , 2010 ;  Kido  etal. , 2014 ;  Fornazari  etal. , 2019 ). The results about which specific changes of junctional zone peristalsis are caused by which type of fibroids seem conflicting.  Nishino  etal.  (2005)  found that only submucosal fibroids, not intramural or subserosal fibroids, cause interruption or even total loss of uterine peristalsis ( Nishino  etal. , 2005 ), while,  Kido  etal.  (2014)  showed that in women with symptomatic fibroids, the presence and frequency of peristalsis and the cervix-to-fundus direction of contractions were significantly decreased, without relationship to the fibroid location or other fibroid characteristics ( Kido  etal. , 2014 ).  Yoshino  etal.  (2010)  demonstrated that in women with intramural fibroids, both low- and high-frequency peristalsis are present, yet pregnancy is significantly more likely to occur in the low-frequency group ( Yoshino  etal. , 2010 ). Furthermore, submucosal fibroids have a significantly thicker pseudocapsule, like a neurovascular bundle surrounding the fibroid, compared to both intramural and subserosal fibroids ( Tinelli  etal. , 2018 ). Hypothetically, this pseudocapsule could play a role in infertility, since it has a recently discovered neuroendocrine function, possibly influencing junctional zone peristalsis ( Tinelli  etal. , 2018 ). Nevertheless, research has shown that treatment of fibroids, such as uterine artery embolization, high-intensity focused ultrasound or myomectomy, can restore the presence, direction, frequency, and coordination of junctional zone peristalsis ( Kido  etal. , 2011 ;  Yoshino  etal. , 2012 ;  Fornazari  etal. , 2019 ;  Qu  etal. , 2019 ). Moreover, Yoshino  etal.  (2012) found that not only the frequency of junctional zone peristalsis was normalized after myomectomy, but the pregnancy rate also was increased (n = 15, pregnancy rate: 40%) ( Yoshino  etal. , 2012 ). To conclude, fibroids can disturb junctional zone peristalsis, probably hampering the journey of sperm cells and oocytes through the genital tract or resulting in implantation failure. Myomectomy might restore junctional zone peristalsis, however, studies reporting on this are scare ( Yoshino  etal. , 2012 ). While studies about obstetric outcomes after myomectomy of submucosal fibroids generally report a positive effect of the procedure ( Klatsky  etal. , 2008 ;  Pritts  etal. , 2009 ;  Bosteels  etal. , 2018 ;  Don  etal. , 2022 ), we still need to elucidate whether this effect is in part because of the normalization of junctional zone peristalsis.\n\nToday it is known that human foetal development is far from a sterile event ( Green  etal. , 2015 ;  Chen  etal. , 2017 ). The fourth hypothesis relates to the influence of fibroids on the vagino-uterine microbiome, since fibroids may induce inflammation and changes in local nutrients, as well as heavy and prolonged menstrual bleeding, all of which influence the vagino-uterine microbiome. A microbiome was found not only in the vagina and endocervical canal, but also in the uterine cavity and fallopian tubes, and even the intrafollicular environment can have its own active microbiome ( Franasiak and Scott, 2015 ). Recent reviews highlight the influence and relevance of the vagino-uterine microbiome to female fertility ( Punzón-Jiménez and Labarta, 2021 ;  Vitale  etal. , 2021 ). The cervicovaginal microbiota is dominated by  Lactobacilli  species and changes in this dominance may influence the reproductive potential ( Huttenhower  etal. , 2012 ;  Franasiak and Scott, 2017 ;  Kroon  etal. , 2018 ). Studies investigating the endometrial microbiome show heterogeneous and conflicting results.  Sola-Leyva  etal.  (2021)  reported endometria with prevalent Clostridium and Staphylococcus (8–13%) and low  Lactobacilli  species (<1%) ( Sola-Leyva  etal. , 2021 ), while other studies showed  Lactobacillus- dominated endometrial samples (>90%) ( Franasiak  etal. , 2016 ;  Moreno  etal. , 2016 ). Moreover,  Moreno  etal.  (2016)  showed that for about one in five endometrial samples, the microbiota was significantly different from that identified in the corresponding vaginal sample ( Moreno  etal. , 2016 ). In women with non- Lactobacillus -dominated microbiota in the endometrial fluid, a significant decrease in implantation, pregnancy, ongoing pregnancy, and live birth rates was found compared to women with a  Lactobacillus -dominated microbiota ( Moreno  etal. , 2016 ). Gynaecological diseases, like endometriosis, ovarian or endometrial cancer and polycystic ovary syndrome, have been shown to influence the microbiome in the female reproductive tract ( Punzón-Jiménez and Labarta, 2021 ). Only two studies have investigated the microbiome in women with fibroids.  Chen  etal.  (2017)  found that in women with fibroids,  Lactobacillus  species were more often present in vaginal and cervical samples, while  L. iners  was more often present in the cervical canal, compared to women without fibroids ( Chen  etal. , 2017 ).  Winters  etal.  (2019)  found different bacterial profiles in vagina and endometrium samples of patients with fibroids, and the endometrium samples were not dominated by  Lactobacillus , but  Acinetobacter, Pseudomonas, Comamonadaceae , and  Cloacibacterium  ( Winters  etal. , 2019 ). Moreover, these authors questioned the observations in other studies of a dominant  Lactobacillus  presence in the upper reproductive tract, since these endometrium samples were generally collected transcervically, and  Winters  etal.  (2019)  collected their samples after hysterectomy. Unfortunately, the fibroid characteristics or associations with fibroid-related abnormal uterine bleeding were not reported. In addition, validation of the results is required in larger prospective cohort studies. It is important to note that the microbiome comprises more than a simple accumulation of bacteria and in many cases it is formed by complex 3-dimensional lattices, also described as the biofilm ( Franasiak and Scott, 2015 ). The relationship between the reproductive tract and its biofilm might be more complex than only the mere absence, presence, or relative concentration of various species of bacteria ( Benner  etal. , 2018 ;  Punzón-Jiménez and Labarta, 2021 ). Additionally, the question remains, what a kind of healthy baseline microbial state of the uterus is and whether ‘correcting’ the microbiome or biofilm to a more baseline state will positively impact reproductive outcomes. In our opinion, this question needs to be answered first, and afterwards research needs to be done in patients with fibroids specifically to clarify what these conditions do to the vagino-uterine microbiome and whether treatment like myomectomy can restore a healthy and receptive vagino-uterine microbiome.\nThe fifth hypothesis is that fibroids could disturb levels of inflammation and autophagy, resulting in a detrimental environment for implantation. It is clear that the female reproductive tract has a symbiotic relationship with its microbiome, and the detection of these microbes by the immune system is followed by a complex dialogue involving its adaptive and innate components ( Agostinis  etal. , 2019 ). The uterine immune system is very unique, due to its adaptation to hormonal stimuli during the menstrual cycle and its toleration of the semi-allogeneic foetus ( Agostinis  etal. , 2019 ). Immune cells show a distinctive pattern of changes throughout the phases of the menstrual cycle, and during implantation and pregnancy. Located in the endometrium, CD68+ macrophages can induce pro-inflammatory cytokines, and these are suggested to have a significant role in fertility, since they support preparation of the endometrium during the window of implantation ( Thiruchelvam  etal. , 2013 ). Macrophage numbers significantly increase during the secretory phase, especially at the site of implantation ( Agostinis  etal. , 2019 ). Supporting the role of local inflammation in fibroid pathogenesis, it is reported that in patients with fibroids, CD68+ macrophages exist in abundance at the myometrial site of the fibroid, compared with that observed in distant autologous endometrium ( Protic  etal. , 2016 ;  AlAshqar  etal. , 2021 ). These macrophages produce various growth factors, including transforming growth factor β (TGF-β), which attracts even more macrophages ( AlAshqar  etal. , 2021 ). In women without fibroids, natural killers (NK) cells are scarcely scattered in the stroma of the functional layer of the myometrium during the proliferative phase; only during ovulation does their number dramatically increase until a couple days before menstruation ( Agostinis  etal. , 2019 ). Low levels of interleukin (IL)-11 are associated with decreased numbers of NK cells, and IL-11 is decreased during the window of implantation in patients with fibroids ( Ikhena and Bulun, 2018 ). This reduction in IL-11 may therefore result in implantation failure in these women, however, further research is needed to assess this association. A summary of the above two processes is depicted in  Fig. 2 . Local endometrial inflammatory changes may contribute to the observed structural changes in patients with fibroids, like glandular atrophy, ulceration, and elongated glands, resulting in implantation failure, early pregnancy loss, and infertility ( Mukhopadhaya  etal. , 2007 ;  Somigliana  etal. , 2007 ;  Zepiridis  etal. , 2016 ;  Roșu  etal. , 2021 ). Whether or not a myomectomy procedure could subsequently re-establish the normal endometrial inflammatory state and allow the repair of the structural endometrial changes still needs to be clarified.\nTwo possible inflammatory routes how fibroids could negatively influence implantation and cause infertility.  TGF-β, transforming growth factor β.\nAutophagy comprises a collection of processes that facilitate cells to recycle and digest harmful or surplus contents in their cytoplasm, such as damaged organelles, proteins, macromolecules, or microorganisms ( Parzych and Klionsky, 2014 ;  Andaloussi  etal. , 2017 ). Recent evidence shows that basal autophagy is essential to maintain endometrial homeostasis and mediate the menstrual cycle and implantation ( Popli  etal. , 2022 ). Additionally,  invitro  and  invivo  studies in mice have shown that autophagy is crucial to establish the process of decidualization, which is indispensable for successful implantation ( Popli  etal. , 2022 ). Disturbed levels of autophagy can lead to endometrial pathologies like fibroids, in which decreased expression of ATG4D is shown to promote fibroid growth ( Andaloussi  etal. , 2017 ;  Popli  etal. , 2022 ). Additionally, high expression of fibroblast activation protein (FAP) is associated with fibroids. Uterine fibroblast proliferation is stimulated by oestrogen, as this induces expression of FAP and extracellular matrix components (fibronectin, laminin, collagen I), and secretion of growth factors (TGF-β and IGF-1) ( Luo  etal. , 2014 ). Whether or not these alterations which are involved in fibroid development and growth, are simultaneously involved in impaired autophagy in the endometrium, leading to implantation failure, is yet to be elucidated. To conclude, more fundamental research is needed to verify whether fibroids alter autophagy in the endometrium and whether this contributes to fibroid related infertility.\nIn our sixth hypothesis, we describe the influence of fibroids on different molecular pathways that could result in a detrimental environment for implantation, as illustrated in  Fig. 3 . Homeobox genes, specifically homeobox A10 (HOXA10) and homeobox A11 (HOXA11), are transcription factors in the endometrium which are important during implantation ( Ikhena and Bulun, 2018 ;  Roșu  etal. , 2021 ). In women with submucosal fibroids, HOXA10 and HOXA11 are significantly decreased in the endometrium, especially in the endometrium overlying the submucosal fibroid ( Rackow and Taylor, 2010 ;  Cakmak and Taylor, 2011 ). Also, in infertile women with intramural fibroids (without cavity distortion), a lower expression of both HOXA10 and the cell adhesion molecule E-cadherin was found during the window of implantation compared to that in fertile women without fibroids ( Makker  etal. , 2017 ). Similarly,  Unlu  etal.  (2016)  found that in infertile women with intramural fibroids, there is reduced expression of HOXA10 and HOXA11, which significantly increases after myomectomy. Conversely, this effect was not seen in women with submucosal fibroids ( Unlu  etal. , 2016 ). Bone morphogenetic protein type II (BMP2) regulates HOXA10 expression, and thus BMP2 increases endometrial receptivity ( Ikhena and Bulun, 2018 ). However, fibroids produce high amounts of TGF-β3, which downregulates the BMP-receptor, resulting in impaired decidualization and implantation ( Doherty and Taylor, 2015b) . Decreased levels of glycodelin and IL-10 have also been described in uterine flushings of women with fibroids, and since these are also involved in the implantation process, the changes could alter this process ( Donnez and Dolmans, 2020 ;  Roșu  etal. , 2021 ). Additionally, as described above, IL-11 is involved in the regulation of trophoblast invasion and in the presence of fibroids, IL-11 levels are reduced ( Donnez and Dolmans, 2020 ). Furthermore, in uterine flushings and in serum of women with fibroids, tumour necrosis factor (TNF)-α level was reported to be increased. As TNF-α is involved in preimplantation development of the embryo, immunological pregnancy loss and regulation of trophoblast invasion, changes in its expression could result in implantation failure and infertility ( Roșu  etal. , 2021 ). These findings suggest that fibroids induce multiple changes in molecular pathways involved in implantation, however, further studies are required to outline all of the affected pathways in patients through which fibroids may cause implantation failure.\nA summary of possible molecular routes how fibroids could negatively influence implantation and cause infertility.  BMP2, bone morphogenetic protein type II; HOXA10/11, homeobox A10/11; IL-10/11, interleukin-10/11; TGF-β3/R, transforming growth factor β3/receptor; TNF-α, tumour necrosis factor-α.\nA recent published review underlines how aberrant endometrial angiogenesis and disturbed vessel maturation caused by fibroids may result in both abnormal uterine bleeding and infertility ( Don  etal. , 2023a ;  Middelkoop  etal. , 2023 ). As mentioned above, alterations in the BMP pathway can negatively influence endometrial decidual differentiation and may result in implantation failure ( Sinclair  etal. , 2011 ;  Doherty and Taylor, 2015a) . BMP activates the downstream SMAD proteins and regulates the TGF-β superfamily, which are involved in several angiogenic activities like endothelial cell differentiation and vessel maturation ( Sinclair  etal. , 2011 ;  Richards  etal. , 2017 ). Two studies found decreased expression of BMP and its receptors in the endometrium of women with fibroids ( Sinclair  etal. , 2011 ;  Doherty and Taylor, 2015a) . Another study showed that in patients with heavy menstrual bleeding, BMP7 was increased, although some patients without fibroids were included in this study ( Richards  etal. , 2017 ). As BMP7 is part of the BMP family and also plays a role in endometrial differentiation, this could have an effect on embryo implantation as well as tissue integrity maintenance during the receptivity window ( Middelkoop  etal. , 2023 ). Endometrial nitric oxide synthase (eNOS), the predominant isoform of nitric oxide in the human endometrium, is also an important angiogenic factor involved in vasodilatation and permeability of endothelial cells and it is induced by vascular endothelial growth factor (VEGF), a major initiator of angiogenesis ( Griffioen and Molema, 2000 ;  Wong  etal. , 2012 ). In infertile women with fibroids higher levels of eNOS were found compared to normal controls; while eNOS stimulates myometrial quiescence during pregnancy, overexpression can induce cellular apoptosis and/or impair endometrial and myometrial functions ( Novin  etal. , 2018 ).\nFibroids, especially those close to the uterine cavity, can restrict endometrial blood flow ( Donnez and Dolmans, 2020 ). Several studies have shown that good endometrial blood flow on the day of the embryo transfer during assisted reproductive treatment is associated with high rates of pregnancy, supporting the idea that alterations in endometrial blood flow can affect blastocyst implantation ( Mercé  etal. , 2008 ;  Kim  etal. , 2014 ). One study found that in women with fibroids and a lower uterine artery pulsatility index and resistance index, conception rates were significantly lower, suggesting that inadequate blood supply of the endometrium due to fibroids can impair implantation ( Ng and Ho, 2002 ). These findings indicate that fibroids do alter angiogenic factors and blood flow in the endometrium, however, many of the factors have been studied only in single studies and more studies are needed to verify these outcomes. This is an essential first step before it is possible for future research to identify key targets for possible therapy or evaluate whether myomectomy can restore normal endometrial angiogenesis.\n\nAs discussed above, fibroids can negatively influence embryo implantation. Studies have shown that patients with fibroids can also experience a number of problems during pregnancy, including miscarriage or early pregnancy loss ( Khaund and Lumsden, 2008 ;  Klatsky  etal. , 2008 ). A number of studies have shown higher miscarriage rates in the first and second trimester in women with fibroids ( Benson  etal. , 2001 ;  Salvador  etal. , 2002 ;  Klatsky  etal. , 2008 ;  Majeed  etal. , 2011 ;  Saravelos  etal. , 2011 ). The exact miscarriage rate in women with fibroids varies widely between these studies, and this could be due to heterogeneous inclusion criteria regarding fibroid characteristics, notably size, number, and location.  Benson  etal.  (2001)  even found a nearly 2-fold increase in miscarriage rate in women with fibroids, and additionally the miscarriage rate in women with multiple fibroids was significantly higher compared to women with a single fibroid ( Benson  etal. , 2001 ). While  Benson  etal.  (2001)  did not find an effect of fibroid location, it is believed that submucosal fibroids in close proximity to the placenta are likely to be associated with first trimester blood loss and also miscarriage ( Khaund and Lumsden, 2008 ). We believe this could be because submucosal fibroids seem to affect junctional zone peristalsis and molecular changes in the endometrium, as we postulated in our hypotheses. While multiple cohort studies about the miscarriage rate after IVF/ICSI in women with fibroids showed no significant difference to that of women without fibroids, a trend towards a higher miscarriage rate was notable ( Vimercati  etal. , 2007 ;  Klatsky  etal. , 2008 ;  Bozdag  etal. , 2009 ). This could be due to the fact that these studies were underpowered; moreover one control group of women without fibroids but receiving IVF/ICSI was already at a higher risk of miscarriage compared to patients not receiving IVF/ICSI ( Klatsky  etal. , 2008 ). Additionally, these studies only included patients with a single fibroid, while  Gianaroli  etal.  (2005)  found a significantly higher miscarriage rate in women with one or multiple fibroids after IVF compared to patients without fibroids, which underlines the effect of multiple fibroids ( Benson  etal. , 2001 ;  Gianaroli  etal. , 2005 ). Conversely, a recent meta-analysis of 1394 pregnant women with fibroids versus >20 000 pregnant women without fibroids, and excluding women with recurrent pregnancy loss or infertility care, found no association between fibroids and the risk of miscarriage ( Coutinho  etal. , 2022 ). Therefore, the current data are conflicting and do not answer the question of whether there is or is no association between fibroids and miscarriage. Future studies examining this issue should therefore focus on a large cohort of the general obstetric population in which patients with and without fibroids are compared. Preferably, in such a study the fibroids should be differentiated according to their characteristics (size, number, location, and vascularization) and there should be correction for potential confounders.\n\nInfertility in patients with fibroids may have various causes. We have postulated seven hypotheses demonstrating mechanisms by which fibroids could cause infertility. The first states that fibroids can cause pelvic pain, dyspareunia, and prolonged and heavy menstrual bleeding, which can disturb sexual arousal and frequency of intercourse, resulting in a reduced probability of conception. The second and third hypotheses state that fibroids could negatively influence oocyte and sperm transport by disturbing myometrial peristalsis of the junctional zone, and/or by mechanical compression on the interstitial part of the fallopian tubes or deformation of uterine cavity. Finally, fibroids could cause a detrimental environment for implantation in five different ways: again by disturbing myometrial peristalsis of the junctional zone, and additionally by altering the vagino-uterine microbiome, affecting the levels of inflammation and autophagy, stimulating molecular changes in the endometrium, and/or disturbing angiogenesis and the endometrial blood supply.  Table 2  shows the future perspectives for the seven hypotheses, as we note the unanswered questions. It is essential to remember that not all fibroids cause symptoms, and the association of infertility and fibroids still needs more research to expose its specific relationship, especially keeping in mind that the effect of fibroids can be dependent on their location, number, vascularization, and molecular influence. We advocate for systematic myometrial assessment during ultrasounds, not only when patients present with gynaecologic complaints indicating fibroids, but also when presenting with infertility. Moreover, fibroid screening is not incorporated in routine ultrasonic checks during pregnancy and therefore is easily missed. To encourage further research about fibroids, it is essential to integrate systematic myometrial assessment, including standard registration of fibroid characteristics (number, size, FIGO location, and vascularization) before, during, and after pregnancy, particularly in case of miscarriages, malpresentation, or premature delivery. This could provide more insight into the disease prevalence and severity, and help in developing research investigating the effect of fibroids on fertility and reproductive outcomes.\nSummary of questions for future research of the seven postulated hypotheses.\n\nFibroids are the most common benign tumours and not all patients with fibroids suffer from infertility while not all infertile patients have fibroids. However, growing numbers of studies underline the possible negative effect of fibroids on fertility and reproductive outcomes. Insight in the underlying mechanisms provides options for the development of targeted therapy and to increase the effect of fibroid surgery due to improved patient selection of who will benefit from a fertility sparing myomectomy. Structural and routinely assessment of fibroids, sizes, localization, and vascularization in women who want to become pregnant and in women with infertility before possible surgery is needed to develop prediction models and to gain insight in the underlying mechanisms of fibroid related infertility.\n\nAll authors approve the publication of this version. They all participated in the conception and drafting of the manuscript, including the interpretation of data from the literature.","source_license":"CC0","license_restricted":false}