Compaction and blastulation after thawing and overnight culture of frozen day 3 embryos are reliable predictors of clinical pregnancy

Abstract

OBJECTIVE: Single embryo transfer (SET) has become daily practice in Belgium in view of a government funded program to reduce multiple pregnancies. This attitude tends to extend now to transfer of frozen embryos. Characterization of the single best frozen-thawed embryo for transfer has become a necessity. We analysed impact of embryo stage before freezing, cell loss and embryo development between thawing and transfer on clinical pregnancy rates in single frozen embryo transfers. DESIGN: Retrospective analysis of 526 single frozen day 3 embryo transfers carried out between January 2004 and October 2007. MATERIALS AND METHODS: Day 3 embryos with at least 6 blastomeres and ≤20% fragmentation were frozen using a slow-controlled DMSO protocol. Embryos surviving thawing were cultured overnight in Medicult BlastAssist medium. Embryo stage before freezing, immediately after thawing and at the moment of transfer was recorded. Clinical pregnancy was defined as a pregnancy with at least one gestational sac at seven weeks of gestation. Pregnancy rates were compared using Chi-squared test (p< 0.05). RESULTS: The overall clinical pregnancy rate was 20.0% (105/526). Embryo stage before freezing did not influence clinical pregnancy rates: 18.9% (14/74) for 6-cell embryos, 23.7% (27/114) for 7-cell embryos, 17.6% (43/245) for 8-cell embryos and 22.6% (21/93) for >8 cell embryos. The clinical pregnancy rate after transfer of frozen embryos with 100% intact cells was 19.9% (76/382). This was not significantly different compared to embryos with 1 blastomere (23.3%, 20/86) or 2 blastomeres (17.6%, 6/34) damaged. Three clinical pregnancies were obtained in the group of embryos with > 2 cells damaged (3/24, 12.5%) originating from transfer of embryos with 50% blastomere damage. Significantly higher clinical pregnancy rates were obtained after transfer of embryos with signs of compaction (71/317, 22.4%, p<0.001) or blastulation (21/62, 33.9%, p<0.0001) than embryos with >8 cells but without compaction (8/109, 7.3%). Embryos with ≤8 cells at transfer had a clinical pregnancy rate of 13.2% (5/38). CONCLUSIONS: Compaction and blastulation after thawing and overnight culture of frozen day 3 embryos are reliable predictors of clinical pregnancy. Loss of 1 or 2 blastomeres did not have a negative impact on clinical pregnancy rates. It is possible to obtain a clinical pregnancy after transfer of embryos with up to 50% of cell loss.