The impact of blastomere survival rates on developmental competence of cryo-thawed Day 2 embryos

The aim of the present study is to investigate the effect of embryonic blastomere loss, following cryopreservation and thaw of Day 2 embryos in an assisted reproductive technology (ART) setting, on pregnancy outcome and fetal development.

This is a retrospective analysis performed on 3553 slow frozen-thawed Day 2 embryos, of all IVF/ICSI thawing cycles carried out during the 11 year study period. Of these thawed embryos, 628 underwent SET on Day 3 of embryo development. We measured the influence of several laboratory parameters on blastomere loss after thaw including: cell stage at cryopreservation, blastomere loss post-thaw, ability to resume mitosis and the rate of overnight cleavage.

There is an association between cell number on day of freeze and embryonic survival post-thaw; 3 cell (77.4%), 4 cell (92.1%), 5 cell (81.4%) and 6 cell (86.5%) embryos (p<0.05). We found a significant association between the rate of overnight cleavage and positive hCG and implantation rate (p-value <0.05), while there is no association with live birth rate (p-value 0.242). Embryos with 100% blastomere survival have significantly higher cleavage rates, positive hCG, implantation and live birth rates than embryos which experienced blastomere loss (p<0.05). However, blastomere survival has no impact on miscarriage rate or the observed newborn birth weight (3.85 ± 0.77 kg).

In the present study we demonstrate, for the first time, that although it is optimal to select an embryo with 100% blastomere survival, transfer of an embryo with ≥ 50% blastomeres intact post-thaw does not influence the development of the baby, as indicated by weight at birth.