Effect of developmental stage of embryos at freezing on live birth outcomes after frozen embryo transfer.

Purpose Our objective was to evaluate the birth outcomes of FET from different developmental stage of embryo at freezing (Day 3, Day 5 and Day 6). Methods All vitrified-warmed day 3 (D3) and day 5/6 (D5, D6) embryo transfers during the time period from January 2013 until December 2015 were included in the study. The Birthweight, Low birthweight (LBW), sexual distinction, gestation and for singleton births and twins conceived by FET were compared retrospectively between three cryopreservation strategies utilizing either D3, D5 or D6 embryo freezing. All embryos were vitrified by Dimethylsulfoxide (DMSO), Ethylene glycol (EG) and Sucrose used as Cryoprotectants and using Cryoleaf as the receptacles. Results A total of 993 infants from 771 women. The length of embryos storage was 3-38 months, with a median of 8 months. For singleton, birthweight from D5 and D6 blastocysts transfers were significantly heavier than for D3 cleavage-stage embryos transfers (P =0.0065 P =0.0006). For twins, birthweight from D6 blastocysts transfers were significantly heavier than for D5 blastocysts transfers (P =0.0044), and children born after D3 were at a signigicantly increased risk of being born a LBW. Conclusions Birthweights from FET are influenced by developmental stage of embryos at freezing, which were lower for cleavage-stage embryos transfer than blastocysts transfer after FET in singletons. The birth rate of LBW infants was higher in the twins. The study was small so there may be other factors than cryopreservation which affected outcomes. A higher sample size or a multi-centre prospective randomized design could be used in future studies to corroborate the current findings.