Do blastocysts which form on day 4 have an altered pregnancy potential? A narrative review

The clinical pregnancy rate for assisted reproductive technology (ART) has remained static for several years. Although the reasons for this are multifactorial, reliable selection of the single optimal embryo for transfer remains a challenge. Embryo selection currently relies on visual assessment, with judgements based on cellular morphology and timing of developmental milestones. Accepted ART practice considers that embryos should reach blastocyst stage on day 5 post oocyte insemination, and this is assumed to be the optimal day on which to utilize the embryo. However, the advent of time-lapse imaging has revealed a variable frequency of blastocyst formation, ranging from day 4 to day 7. Despite current practice not involving routine assessment on day 4, emerging evidence suggests that embryos which reach blastocyst stage and are utilized on day 4 may have an improved chance of pregnancy. Reducing time in vitro could also have benefits for ongoing pregnancy and offspring health. This review provides a clinical perspective on the relationship between the day of blastocyst formation and pregnancy outcomes. Reflecting on this evidence will allow clinical professionals to consider both monitoring and utilizing blastocysts on day 4 in order to improve pregnancy success outcomes after ART.