Outcomes of embryo vitrification at different developmental stages: Evaluation of 2412 warming cycles

Advances in cell culture media have led to a shift from cleavage stage embryo transfer to blastocyst stage transfer. Extended embryo culture to the blastocyst stage provides some theoretical advantages and disadvantages. There has been controversy. This study is sought to evaluate the clinical outcomes of vitrified-warmed cleavage-stage and blastocyst-stage embryo transfers in patients undergoing Artificial Reproductive Technique treatments.

The study was performed on 2740 women undergoing frozen embryo thawing transfer. Patients' basic situation, status of frozen embryo transfer cycle, clinical pregnancy rate, early abortion rate, sex ratio of birth, and birth weight were retrospectively analyzed. We compared the main clinical results of recovery of frozen embryo on the cleavage stage and blastocyst stage. Furthermore, we evaluated the clinical outcomes of blastocyst cryopreservation on Day 5, 6, or 7 after oocyte retrieval according to the day of blastocyst expansion were evaluated.

The implantation ratio of cleavage stage embryos was 21.62% compared with 43.52% on D5 (P < .05). The D5, D6, and D7 implanting rates were statistically different. The pregnancy rates were 57.56%, 51.76%, and 35.95% versus 37.79%, respectively for embryos cryopreserved on D5, D6, D7, and D3. The ectopic pregnancy rate and early abortion rate were statistically different between D5 and D3. The sex ratio, the birth weight, and birth defect were not statistically different among the four groups.

Blastocyst transfer achieved a higher implantation rate than vitrified cleavage stage embryo and decreased ectopic pregnancy rate. With increased incubation days before expansion blastocyst formed, the implantation rate is reduced and the early abortion rate increases.