{"paper_id":"f04bea91-062b-4a1c-88e7-d0912fffdd37","body_text":"Since the beginning of assisted reproduction techniques (ART), morphology has been used as a standard method for embryo selection, generating comparable clinical outcomes among laboratories ( Ludwig  et al ., 2000 ). Currently, several laboratories are culturing embryos until the blastocyst stage, and the best morphology-embryo is selected for embryo transfer ( Scott  et al ., 2000 ).\nThe Time-Lapse (TL) system in ART was first described by  Payne  et al . (1997) , who reported morphokinetic events in early embryo development stages, some of them were: second polar body extrusion, pronuclear formation (maternal and paternal) and cellular cleavage. Morphokinetics enables to compare time frames and dysmorphic events during embryo development ( Payne  et al ., 1997 ). With all this information, algorithms have been proposed with the aim to increase implantation rates in ART ( Sakkas  et al ., 1998 ;  Gardner  et al ., 2015 ).\nEmbryo culture in TL incubators enabled us to continuously monitor embryo development, without interfering with culture conditions (temperature, gas concentrations and pH); thus, decreasing the negative impact on embryo development ( Kahraman  et al ., 2020 ) and minimizing the negative effect of standard embryo evaluation under conventional microscopes.\nThe clinical effectiveness of TL systems is still controversial. A meta-analysis concluded that the TL was associated with a significantly higher ongoing pregnancy, live birth rate and significantly lower early pregnancy loss in comparison to the conventional incubator and morphological selection ( Pribenszky  et al ., 2017 ); whereas a recent Cochrane review concluded that there are insufficient evidence to support a higher live birth rate when the TL system is used with the aid of software assessment ( Armstrong  et al ., 2018 ). Furthermore, similar meta-analyses have demonstrated some clinical benefits using TL ( Armstrong  et al ., 2018 ;  Chen  et al ., 2017 ). This study aimed to describe how morphokinetic parameters differ according to patient characteristics and culture conditions during embryo development.\n\nIt is widely accepted that older women have diminished ovarian reserve and poor oocyte quality ( Navot  et al ., 1991 ). Additionally, old patients have higher chances to produce aneuploid embryos, which reduce pregnancy rates ( Grøndahl  et al ., 2017 ).\nRetrospective studies concluded that there are no significant differences in morphokinetic parameters ( Tables 1  and  2 ) during cleavage stages between the embryos from advanced maternal age and those of younger women ( Hickman  et al ., 2013 ;  Silva  et al ., 2015 ;  Gryshchenko  et al ., 2014 ;  Warshaviak  et al ., 2019 ). By contrast, two retrospective studies reported a significantly faster cleavage in some kinetic parameters in younger patients when compared to older ones ( Akarsu  et al ., 2017 ;  Dal Canto  et al ., 2021 ). Nevertheless, these differences are not evident in patients aged ≥ 37 years ( Dal Canto  et al ., 2021 ).\nDefinition of time-lapse morphokinetic parameters.\nSummary of the studies highlighting the morphokinetic variables with statistically significant differences.\nNote: NA: not available; d2/3/5: day 2/3/5; other abbreviations in  Table 1 .\nOn the other hand, morphokinetics is faster in later stages (tM, tSB, tB and tHatched) in younger patients (<36 years) than their counterparts; it becomes slower with increasing maternal age ( Hickman  et al ., 2013 ;  Silva  et al ., 2015 ).\nBMI is defined as the body weight in kilograms divided by the square of height in meters; it is universally expressed in units of kg/m 2 , used to broadly categorize a person as underweight (<18,5 kg/m 2 ), normal weight (18.5 to 24.5 kg/m 2 ), overweigh (25 to 30 kg/m 2 ) or obese (over 30 kg/m 2 ) ( WHO, 2020 ).\nObese women need longer time to become pregnant ( Gesink Law  et al ., 2007 ;  Wise  et al ., 2010 ) and their risk of infertility is almost three times higher compared to non-obese women ( Zaadstra  et al ., 1993 ). Obese women who underwent fertility treatments have lower number of retrieved and mature oocytes, poorer embryo quality ( Bellver  et al ., 2013 ), lower fertilization ( Krizanovská  et al ., 2002 ) and increased miscarriage rates ( Rittenberg  et al ., 2011 ).\nClassical morphological evaluation does not seem to be useful for analyzing the impact of obesity on embryo quality ( Bellver  et al ., 2010 ;  Shah  et al ., 2011 ). However, morphokinetic parameters showed differences. A recent study, found that t5 and t8 were delayed in obese and in overweight women in relation to the normal-weight group ( Bartolacci  et al ., 2019 ); while another study reported that embryos generated from obese and normal-weight infertile women showed similar cleavage patterns, but both groups were slower in early cleavage stages (t2, t3, t4 and t5) than embryos from fertile donors (control) ( Bellver  et al ., 2013 ). In contrast, other studies reported that higher BMI (>25 kg/m 2 ) was associated with earlier occurrence of tM ( Leary  et al ., 2015 ) as well as earlier tPN, tPNf, t2, t4, t6, t7 and t8 and S2 than normal-weight women ( Mumusoglu  et al ., 2017 ).\nThere is no strong evidence that underweight women are associated with menstrual abnormalities or infertility. However, TL systems have enabled to study the morphokinetics of these patients. A study found that underweight women presented later t4, t5, t6 and t8 cell stages than their normal-weight, overweight and obese counterparts ( Lammers  et al ., 2013 ). By contrast, a study reported that BMI (low or high) does not impact embryo morphokinetics ( Silva  et al ., 2015 ).\nPCOS affects 5-10% of women of reproductive age. It is considered a heterogeneous disorder. It is identified using the Rotterdam criteria, in which two out of three of the following conditions must be met: the presence of oligo- and/ or anovulation, clinical and/ or biochemical signs of hyperandrogenism and polycystic ovaries.\nMorphokinetic parameters in hyper-androgenic PCOS women had a delay in early stages (from tPNf to t8) compared with embryos from non-PCOS regular cycles, but there was no kinetic difference between normo-androgenic PCOS and control women ( Wissing  et al ., 2014 ). In addition, another report stated that embryos generated from PCOS patients had long developmental timings (tPNf, t2, t3, t4, t5, t6, t7 and t8) compared to normo-ovulatory women ( Tabibnejad  et al ., 2019 ).\nNevertheless, other studies reported a faster development in PCOS and hyper-androgenic patients when compared to embryos from non-PCOS patients during the cleavage stage (t5, t6, t7, t8, t9 and 4 th  cleavage division (from 8-cells to 9 or more cells), time to tSC and tM ( Sundvall  et al ., 2015 ;  Chappell  et al ., 2017 ;  2020 ).\nOn the other hand, a recent paper documented any significant differences in early embryonic development between the PCOS group and control patients. However, we must emphasize that in this study, the embryos were cultured until day 2 ( Tam Le  et al ., 2019 ).\nFinally, the current available data concluded that the implantation rate and the clinical pregnancy rate did not differ between PCOS patients and their control counterparts ( Wissing  et al ., 2014 ;  Sundvall  et al ., 2015 ;  Tabibnejad  et al ., 2019 ;  Tam Le  et al ., 2019 ). Retrospective studies concluded that PCOS patients have a trend towards a lower live birth rate and higher spontaneous abortion rates ( Chappell  et al . 2017 ;  2020 ).\nEndometriosis is defined as the presence of endometrial glands and stroma cells growing outside the uterus ( Burney & Giudice, 2012 ). Endometriosis affects 10-15% of all women in reproductive age (Giudice & Kao, 2004). It is associated with chronic pelvic pain and infertility ( Parasar  et al ., 2017 ), and it is subdivided into four categories: stage I (minimal), stage II (mild), stage III (moderate) and stage IV (severe) ( Johnson  et al ., 2017 ).\nOne study reported that endometriosis influences morphokinetic parameters without considering the severity of the disease; especially in early embryo stages ( Freis  et al ., 2018 ). However, a deeper analysis associated with the endometriosis stages reported that patients with severe endometriosis reached t9 faster than patients with minimal or moderate endometriosis, as well as the control group (patients without endometriosis). Moreover, the synchronicity of the two blastomere divisions within the second cell cycle (s2) was faster in the endometriosis group than in women without endometriosis ( Schenk  et al ., 2019 ).\nOn the other hand, a recent study concluded that embryos from endometriotic patients had different kinetics during cleavage stages, because they had a shorter cc1 and longer tPB, tPNa; as well as s2 than control group patients ( Boynukalin  et al ., 2019 ).\nIts negative impact on fertility has been widely described in women and men, affecting gamete quality and embryo development ( Waylen  et al ., 2009 ). Smoker women have a higher risk of fertilization failure and lower implantation rates ( Gruber  et al ., 2008 ). Furthermore, smoking causes obstetric complications, such as miscarriages, preterm births and premature membrane rupture ( Levis  et al ., 2014 ).\nWe know that smoking affects at least one of the embryological events in the early cleavage stage of development ( Siristatidis  et al ., 2015 ). A study found that embryos from smokers are significantly slower during cleavage stages (t3, t4 and t5), than those from nonsmokers; curiously, there are no kinetic differences between embryos from smokers and nonsmokers that stopped their development ( Fréour  et al ., 2013 ).\n\nEmbryos can be cultured in uninterrupted culture using a single medium throughout the 5/6 days of culture or sequential media, where two media with different composition are used sequentially ( Costa-Borges  et al ., 2016 ).\nStudies with oocytes from donors and patients have not reported differences in morphokinetic parameters between single step media and sequential media in embryo cultures until days 3 or 5/6 ( Basile  et al ., 2013 ;  Costa-Borges  et al ., 2016 ).\nOn the other hand, a study using autologous oocytes suggested that embryos cultured in a single media have faster development from tPNf until t5 than those cultured in sequential media ( Ciray  et al ., 2012 ); while a multicenter clinical trial reported a delay in t7, t8 in embryos cultured in single media ( Hardarson  et al ., 2015 ).\nInterestingly, besides the differences in morphokinetics, the studies did not find differences in implantation rate, ongoing pregnancy rates and live birth between the groups ( Ciray  et al ., 2012 ;  Basile  et al ., 2013 ;  Hardarson  et al ., 2015 ;  Costa-Borges  et al ., 2016 ). However, single media simplifies the logistics of an IVF laboratory, and the embryos can be assessed without disturbing culture conditions through the time-lapse system.\n\nOocytes are inseminated using conventional in vitro fertilization (IVF) or Intracytoplasmic sperm injection (ICSI). When IVF is performed, some sperm selection processes are maintained in vitro. These processes are sperm penetration of the cumulus cells, zona pellucida interaction and fusion of the oolemma, whereas during ICSI a single sperm is injected into a mature oocyte.\nThese two processes are different, and it is not possible to determine exactly when fertilization occurs in IVF. Some time-lapse studies reported a faster development in embryos generated by ICSI than in IVF ( Lemmen  et al ., 2008 ;  Dal Canto  et al ., 2012 ;  Kim  et al ., 2017 ). However, it is a consensus that tPNf can be used as starting point when IVF or ICSI are used as a fertilization method ( Cruz  et al ., 2013 ;  Bodri  et al ., 2015 ).\nWhen tPNf was used as a starting point, there were any observed differences in the cleavage stage (early and late) between the IVF and the ICSI techniques ( Cruz  et al ., 2013 ;  Bodri  et al ., 2015 ), although one of them reported a faster development in the blastocyst stage in IVF- fertilized embryos ( Bodri  et al ., 2015 ). This could be explained because when IVF is performed the semen characteristics are superior to those in ICSI ( Cruz  et al ., 2013 ;  Bodri  et al ., 2015 ). Under this evidence, confounding factors should also be considered when both techniques are compared.\n\nOxygen concentration differs in the female reproductive tract, and it decreases from the fallopian tubes towards the uterus. The concentration ranges from 5-7% to 2% ( Ng  et al ., 2018 ). Oxygen can influence embryo development. It is well documented that embryos cultured under lower oxygen concentration (5%) have more cleavage, good quality and more embryos are cryopreserved, which improved embryo implantation compared to those embryos cultured under higher oxygen concentration (20%) ( Sciorio & Smith, 2019 ;  Van Montfoort  et al ., 2020 ). Similar results were also found in the TL-system. A study reported a low quality of embryos, a lower number of utilized embryos and delays in s3 when the embryos were cultured in high-oxygen concentration (20%) ( Kirkegaard  et al ., 2013b ).\nAdditionally, in the TL-system, the embryos do not need to be removed from the incubator, this minimizes environmental disturbances caused by bench-top incubators. Interestingly, a study reported a higher live birth rate in embryos cultured in the TL-system, when compared to embryos cultured in high-quality K-systems, both using 5% O 2  ( Kalleas  et al ., 2020 ).\n\nCollapse is a separation of all or part of the trophectoderm (TE) cells from the zona pellucida (ZP) during blastocyst growth, the cells could be separated; when more than 50% form the inner side of the ZP (collapse or strong contraction) or less than 50% (weak contraction) ( Marcos  et al ., 2015 ).\nBlastocyst collapse is independent of the insemination technique ( Sciorio  et al . 2020a ; b ) and maternal age ( Bodri  et al ., 2016 ;  Gazzo  et al ., 2020 ). Whereas the relationship between collapse and embryo quality is arguable, because a study did not find a relation ( Marcos  et al ., 2015 ), while two studies proposed a higher incidence of blastocyst collapse in poor quality embryos ( Bodri  et al ., 2016 ;  Sciorio  et al ., 2020b ).\nEmbryos at the blastocyst stage could collapse at least one time. However, not all the blastocyst collapses ( Marcos  et al ., 2015 ;  Bodri  et al ., 2016 ;  Viñals Gonzalez  et al ., 2018 ;  Sciorio  et al ., 2020a ; b ). Retrospectives studies in TL showed that embryos that collapse during their development have lower implantation and pregnancy rates, especially if they were multiple when compared to blastocysts that did not display this event, although there are no relations between collapse duration and decreased implantation rates ( Marcos  et al ., 2015 ;  Bodri  et al ., 2016 ;  Viñals Gonzalez  et al ., 2018 ;  Gazzo  et al ., 2020 ;  Sciorio  et al ., 2020a ; b ).\nEmbryo collapse and aneuploidy has also been investigated. It is reported that collapses were predominantly present in aneuploid embryos; moreover, embryos affected with monosomies have lower numbers of collapses when compared with trisomy or complex aneuploidies ( Viñals Gonzalez  et al ., 2018 ). Even if an euploid embryo has collapses, it has a lower live-birth rate ( Harton  et al ., 2016 ).\nAccording to morphokinetic parameters, embryos without collapse had a slow development during early stages (t2,t3,t4 and tB) compared to those blastocyst which underwent blastocyst collapse ( Marcos  et al ., 2015 ); while other studies reported that collapsed embryos took longer to reach the blastocyst stage ( Gazzo  et al ., 2020 ), and a delayed development during the blastocyst stage ( Bodri  et al ., 2016 ).\n\nFreezing embryos is a common practice. It enables a single embryo transfer, genetics test or freeze all strategy for better endometrium preparation. During vitrification processes, embryos are dehydrated through the addition of cryoprotectants and, consequently, they shrunk. Immediately after warming, the embryos look collapsed, and need more time to recover their initial volume (re-expansion).\nBlastocyst quality evaluation is difficult after the warming process. For that reason, post-warmed culture helps assess the vitrified/warmed blastocyst ( Zhao  et al ., 2019 ). A study using the TL-system reported that re-expansion started as early as 10 minutes and can complete the re-expansion in 2 hours after warming ( Ebner  et al ., 2017 ).\nEmbryos without re-expansion after the warming process have a significantly lower implantation rate when compared with the completely or partially re-expanded blastocyst ( Desai  et al ., 2016 ;  Coello  et al ., 2017 ;  Ebner  et al ., 2017 ). In addition, after warming, the embryos can also have collapses, even more than once. However, it does not seem to affect implantation ( Coello  et al ., 2017 ;  Ebner  et al ., 2017 ). Shorter-duration re-expansion (from start to complete re-expansion) has been documented in those blastocysts that result in pregnancy in comparison to non-pregnant women ( Ebner  et al ., 2017 ).\nFurthermore, a predictive implantation model that evaluated morphology in post-warmed blastocyst has been proposed. The embryos were subdivided into four categories from A to D. If the maximum area values were >14,597mm 2 , the blastocysts were categorized as A or B, depending on whether the initial area values were >9,900mm 2  or < 9,900mm 2 , respectively. Similarly, if the maximum area values were < 14,597mm 2 , the blastocysts were categorized as C or D, depending on the initial area. They found that implantation was 47.3%, 43.7%, 32.8 % and 14.2% for A to D, respectively ( Coello  et al ., 2017 ).\n\nAtypical dynamic embryo behaviors could happen during their early development, and are not maternal-age dependent ( Athayde Wirka  et al ., 2014 ;  Zhan  et al ., 2016 ;  Yang  et al ., 2018 ). Embryos with anomalous divisions are more likely to arrest their development than embryos with normal divisions, especially during their first stages of development ( Lagalla  et al ., 2017 ). Moreover, their embryo potential and implantation rates are adversely affected ( Meseguer  et al ., 2011 ;  Hlinka  et al ., 2012 ;  Hur  et al ., 2018 ;  Yang  et al ., 2018 ). These abnormal behaviors can only be visualized through continuous observation and subsequent time-lapse analysis.\nAlthough, several atypical dynamic embryo behaviors have been described, two have shown to play a major role: Direct Cleavage (DC) and Reverse Cleavage (RC).\nDirect Cleavage is defined as the second cell cycle being shorter than 5 hours, or more than 2 cells originated from a single cell division event. DC can occur during cleavage and their frequency is higher during the first cleaving periods ( Fan  et al ., 2016 ); moreover, it could be present more than once ( Zhan  et al ., 2016 ). It is noteworthy that DC in embryos, especially at earlier stages, strongly correlates with impaired blastocyst formation, implantation and clinical outcome ( Rubio  et al ., 2012 ;  Athayde Wirka  et al ., 2014 ;  Fan  et al ., 2016 ;  Zhan  et al ., 2016 ).\nDC embryos are more often found during IVF cycles than ICSI cycles ( Zhan  et al ., 2016 ), at higher percentages when testicular and epididymal sperm are used ( Kahraman  et al ., 2020 ;  Zhan  et al ., 2016 ). Additionally, DC embryos have 2.5 to 3.1 fold higher likelihood of a multinucleation incidence ( Zhan  et al ., 2016 ).\nLikewise, two retrospective studies suggest that DC embryos can undergo self-correction mechanisms while excluding some cells (with higher incidence of aneuploidies) during the compaction process. Therefore, DC embryos showed a similar euploid rate when compared to non-DC blastocysts on day 5 ( Zhan  et al ., 2016 )4. Nevertheless, this mechanism is significantly lower in older patients (>39 years) ( Lagalla  et al ., 2017 ).\nReverse Cleavage is defined as either blastomeres rejoining after complete separation, or the blastomere fails to separate. It could occur during the cleavage stage, with major incidence during the third cycle of the mitotic division (5 to 8 cells) ( Liu  et al ., 2014 ). The mean time of occurrence reported was 46.07h ( Quera  et al ., 2014 ) and between 24 to 136h ( Hickman  et al ., 2013 ).\nIn addition, RC affect the blastocyst stage development ( Desai et al., 2018 ). Embryos with RC have significantly lower good-quality embryos on day 3 ( Liu  et al ., 2014 ), and consequently a lower blastocyst formation rate (Yang  et al ., 2018). Furthermore, RC embryos implanted less than their counterparts ( Liu  et al ., 2014 ), especially if two or more abnormal cleavages were present in a single embryo, increasing the likelihood of the embryo being aneuploid ( Desai  et al ., 2018 ).\n\nA meta-analysis concluded that Time-lapse embryo monitoring may help predict the ploidy of embryos ( Swain, 2013 ), even though other studies proposed that morphokinetics are not enough ( Reignier  et al ., 2018 ); but it provides valuable information for embryo selection ( Zaninovic  et al ., 2017 ).\nSome studies did not find differences during the cleavage and blastocyst stages of embryo development between euploids and aneuploids ( Stevens  et al ., 2012 ;  Semeniuk  et al ., 2013 ;  Yang  et al ., 2014 ) ( Table 3 ). Conversely, others have reported a development delay in aneuploid embryos during cleavage and blastocyst stages ( Table 3 ) ( Davies  et al ., 2012 ;  Campbell  et al ., 2013 ;  Vera-Rodriguez  et al ., 2015 ;  Chawla  et al ., 2015 ;  Mumusoglu  et al ., 2017 ;  Zhang  et al ., 2017 ;  Huang  et al ., 2019 ).\nSummary of the studies that combine Time lapse embryo culture and PGT-A analysis.\nNote: NA: not available; d2/3/5: day 2/3/5; FISH: fluorescence in situ hybridization; aCGH: array comparative genomic hybridization; qPCR: quantitative polymerase chain reaction; other abbreviations in  Table 1 .\nSimilarly, a prospective study reported that chromosomally normal embryos display strict and tightly clustered cell cycle parameters up to the 4-cell stage ( Chavez  et al ., 2012 ). Other studies found shorter stages in euploid embryos than aneuploid embryos: in s3 ( Bayram  et al ., 2012 ), a shorter compaction ( Melzer  et al ., 2013 ) and early cavitation from first cytokinesis and from t5 ( Hong  et al ., 2013 ), a shorter cleavage s2 and t4 as well during the blastocyst stage tB, tEB and tHed ( Minasi  et al ., 2016 ), a faster development from t2 to blastocyst stage; however, these differences are not present in severe forms of male infertility (less than 1x10 6  spermatozoa /ml) ( Kahraman  et al ., 2020 ).\nIn addition, a retrospective study reported that embryos with trisomy showed very similar kinetics to those of normal embryos, whereas embryos with monosomies fall between complex and trisomy embryos ( Del Carmen Nogales  et al ., 2017 ). Furthermore, embryos with complex chromosomal abnormalities have the shortest division times (t3, t5, cc2, cc3, s2 and t5-t2), which is strongly associated with t3 and t5-t2 (time interval between the 2- and the 5-cell stage) ( Del Carmen Nogales  et al ., 2017 ). Moreover, a study reported that unbalanced chromosomal translocation embryos showed a delay of t4, tSB and s2, and embryos with balanced translocation did not; even a delay in tPNf was seen in embryos with nonviable unbalanced chromosomal translocation, when compared to potentially viable embryos ( Amir  et al ., 2019 ).\nLikewise, there are models proposed to identify embryos more likely to be euploid, based on variables. Some studies have identified that t5-t2, cc3, tSB and tB are good predictor variables ( Basile  et al ., 2014 ;  Campbell  et al ., 2013 ); others have been more specific, a cc3 (t5-t3) >10.00h and t5-t2 > 20.00h ( Chawla  et al ., 2015 ), and blastocyst initiation (tSB>96.2h); progression to expanded blastocysts (tSB>166h) and tEB- tSB >13h in aneuploid embryos ( Desai  et al ., 2018 ) are morphokinetic parameters associated with aneuploidy.\nOn the other hand, other studies demonstrated that the models described above were unable to discriminate between euploid and aneuploid embryos ( Kramer  et al ., 2014 ;  Rienzi  et al ., 2015 ;  Patel  et al ., 2016 ;  Zhang  et al ., 2017 ).\nA single study with 177 embryos reported that female embryos showed earlier cavitation than male embryos, but it did not reach statistical significance ( Serdarogullari  et al ., 2014 ). However, studies with aCGH analysis found a faster development in male embryos. A single study found that male embryos tend towards a faster progression to t4 than female embryos, with 213 embryos included ( Melzer  et al ., 2013 ); while a study with 327 embryos also found a faster development in male embryos. Hence, they proposed an algorithm based on s2 and tM that permits identifying embryos with higher probability of being female ( Bronet  et al ., 2015 ). By contrast, a study evaluated 416 embryos through the FISH technique, and they reported that tPN is significantly faster in female than male embryos; whereas tPNf was significantly faster in male embryos; and the blastulation rate was significantly higher in female embryos ( Zeyad  et al ., 2018 ).\nAs described above, a retrospective study based on 81 live births reported a significantly slower development for tB in male embryos than in female embryos ( Bodri  et al ., 2016 ). Meanwhile, retrospectives studies reported t3, t4 and cc2 earlier in male embryos than those of female embryos ( Huang & Jin, 2017 ); as well as t3,t4,t5 and cc2 ( Huang  et al ., 2019 ). However, only t3 (<14h) was correlated with live birth sex ( Huang  et al ., 2019 ).\n\nTime Lapse provides valuable information and enormous potential to enhance our understanding of embryo development. Considerable knowledge has been accumulated and describes the morphokinetic dynamics during different stages in human embryos. It has been demonstrated that some patient characteristics and culture conditions modified this development pattern. There is a consensus that embryos generated from advanced maternal age have a slow development. Moreover, the current available data about smoking and high-oxygen concentration cultures are scarce, but it also demonstrated a slow development in both groups. On the contrary, any difference in morphokinetics has been described when IVF or ICSI was used as the fertilization method. Similar observations were found when the embryos were culture in single or sequential media.\nOn the other hand, the limited number of studies, the considerable differences in the study designs and patients' characteristics like BMI, PCOS, endometriosis, ploidy and gender made it difficult to draw a conclusion. Given this inconsistency and lack of evidence, more prospective studies and further randomized clinical trials are needed.\nThe association between morphokinetics and ART outcome has important implications in clinical results; where two morphokinetic events have the potential to predict embryo implantation: blastocyst collapse-re-expansion and direct-reverse cleavage seems to have a strong impact on clinical outcomes.","source_license":"public-domain-us","license_restricted":false}