The time has come to reevaluate the fertilization check

Abstract

Embryology has evolved tremendously over the past 40 years owing to improvements in culture systems, cryopreservation, intracytoplasmic sperm injection, and the development of genetic technologies enabling the screening of embryos by means of direct karyotyping. During this time, the fertilization check is one of the few embryology procedures that has persisted relatively unchanged. As blastocyst culture is becoming commonplace, the previous inclination to monitor embryo development daily is quickly becoming a thing of the past. Many labs, even without the aid of time-lapse monitoring, are leaving embryos undisturbed in the incubator after documented fertilization until day 5. The fertilization check, however, remains sacrosanct. It is standard for embryology labs to inspect eggs ∼16–18 hours post insemination to determine if it has normally fertilized. This is a time-consuming step and, because of the need to adhere strictly to timing, adds complexity to the embryology workflow. The purpose of this check is to properly dispose of eggs that did not fertilize normally (1The Practice Committee of the American Society for Reproductive Medicine and the Practice Committee of the Society for Assisted Reproductive TechnologyRevised guidelines for human embryology and andrology laboratories.Fertil Steril. 2008; 90: S45-S59Abstract Full Text Full Text PDF PubMed Scopus (41) Google Scholar). There are at least two reasons to identify eggs that have not fertilized or have fertilized abnormally: 1) these eggs will not produce viable embryos; or 2) the embryos that are produced will result in pregnancies that are at increased maternal or fetal risk. Li et al. (2Li M, Huang J, Zhuang X, Lin s, Dang Y, Wang Y, et al. Obstetric and neonatal outcomes after the transfer of vitrified-warmed blastocysts developing from nonpronuclear (0PN) and monopronuclear (1PN) zygotes: a retrospective cohort study. Fertil Steril 2021;115:110-7.Google Scholar) share their large dataset analysis of obstetrical and neonatal outcomes after the transfer of blastocysts derived from nonpronuclear (0PN) and monopronuclear (1PN) zygotes. In their study, a far smaller fraction of 0PN and 1PN embryos developed to blastocysts compared with 2PN zygotes. Despite this, the zygotes that did develop to blastocysts had reasonable implantation and live birth rates. In fact, the 0PN-derived blastocysts had virtually identical live birth rates to the control 2PN group (35.6% and 35.2%, respectively), and the 1PN success rate was only slightly lower (27.4%). The 1PN zygotes did appear to increase the risk of spontaneous abortion (33.6% vs. 23.8%), although the total number of pregnancies in this group was low (116). The authors are correctly cautious regarding the health of offspring. Although the mean birth weight was not different between groups, a post hoc subanalysis showed a higher proportion of very large for gestational age (VLGA) offspring in the 0PN group (17%) compared with the 2PN group (11%). This last finding will need to be corroborated by others. It would be premature to advise against transferring blastocysts based on this finding. The percentage of VLGA offspring was not listed as a primary end point in the study design. The possibility of a type I error is high in this setting. This manuscript does not conclude that the fertilization check should be abandoned. Rather, it suggests that the finding of no pronuclei or only one pronucleus at 16–18 hours after insemination should not be regarded as definitive evidence of failed fertilization or an abnormal zygote. It is reasonable to continue culture and, should development continue to the blastocyst stage, perform embryo transfer. The authors propose a very reasonable algorithm as to how these embryos should be prioritized. In view of the referenced studies that blastocysts from 0PN and 2PN zygotes have similar chromosomal compositions, it would be reasonable to transfer these 0PN blastocysts if no 2PN blastocysts are available. On the other hand, despite reasonable live birth rates, the authors conclude that based on the apparent increase in miscarriage risk and paucity of information regarding possible uniparental diploidy, transfer of 1PN blastocysts should also necessitate genetic counseling. It is possible to draw a different conclusion from that drawn by Li et al. (2Li M, Huang J, Zhuang X, Lin s, Dang Y, Wang Y, et al. Obstetric and neonatal outcomes after the transfer of vitrified-warmed blastocysts developing from nonpronuclear (0PN) and monopronuclear (1PN) zygotes: a retrospective cohort study. Fertil Steril 2021;115:110-7.Google Scholar) The alternate conclusion is that the fertilization check is an imperfect screening tool with an unknown benefit in the era of extended culture. Although polypronuclear zygotes were not studied, a legitimate argument can be made that the extended culture process might prove adequate to screen out fertilization failures. This check may not confer any advantages to the natural biology of early embryo development. Drawbacks related to increased handling, light exposure, human resource and workflow complexities, and false positives (as shown in the present study) must be considered. This study clearly suggests that failure to identify 0PN and 1PN embryos would not significantly affect success rates. Although we should be concerned about safety, it is reassuring that non–in vitro fertilization (IVF) medically assisted conception procedures, such as intrauterine insemination, do not appear to pose obstetrical or neonatal risks compared with IVF. In addition, intravaginal fertilization and embryo culture (IVC) is increasing in popularity due to availability of a device designed and marketed for this purpose. IVC precludes the fertilization check by design. The initial experience with IVC, albeit small, has not demonstrated significant differences in obstetrical or neonatal outcomes compared with standard IVF with fertilization checks (3Doody K. Broome E. Doody K. Comparing blastocyst quality and live birth rates of intravaginal culture using INVOcell to traditional in vitro incubation in a randomized open-label prospective controlled trial.J Assist Rep Genet. 2016; 33: 495-500Crossref PubMed Scopus (10) Google Scholar). IVF has evolved to be a very complex undertaking with a high success rate but high financial cost. We should consider streamlining care to reduce cost without lowering success or adding risk. The fertilization check might prove to be low hanging fruit. It certainly should not be considered untouchable. Further analysis is necessary. Obstetric and neonatal outcomes after the transfer of vitrified-warmed blastocysts developing from nonpronuclear and monopronuclear zygotes: a retrospective cohort studyFertility and SterilityVol. 115Issue 1PreviewTo evaluate the obstetric and neonatal outcomes after the transfer of vitrified-warmed single blastocysts developing from nonpronuclear (0PN) and monopronuclear (1PN) zygotes. Full-Text PDF