Behind the scenes: Cleavage patterns of 9600 embryos

Abstract This study investigates the incidence of irregular cleavage (IRC) among human embryos and their influence on IVF treatment outcomes. This study was designed as a prospective observational study in a single-centre IVF clinic including 1,001 women who underwent 1,976 assisted reproduction treatments during 2016–2021. Morphokinetics of embryos was analysed and evaluated for the association between IRC and women’s characteristics, treatment characteristics, and pregnancy outcome. We found IRC incidence to be 17.5% (1,689/9,632 embryos). Of these, 85% embryos had one IRC, and 15% had multiple IRC. 35% of IRC events occurred during the embryo's first cell cycle. IRC embryos were found to correlate with male factor (p = 0.01) and higher ICSI rate (p = 0.01). Age, BMI, parity, basal FSH level, stimulation protocol and number of retrieved oocytes did not differ between groups. Embryos with early IRC or more than one IRC event had lower blastulation rate (p = 0.01 and p = 0.01, respectively). Fresh cycles with IRC embryos had a lower clinical pregnancy rate (p = 0.01), and early IRC embryos had a lower live birth rate (p = 0.04) compared to embryos without IRC. Frozen embryo transfer (FET) cycles of blastocyst embryos, with and without IRC, had comparable results. In conclusion, number of abnormal cleavage events and their timing are of great importance for the prognosis of the developing human embryo.