Should embryos developing to blastocysts on day 7 be cryopreserved and transferred: an analysis of pregnancy and implantation rates.

OBJECTIVE: To compare pregnancy rates (PRs) using blastocysts cryopreserved on day 7 with those cryopreserved on days 5 and 6.

DESIGN: Retrospective observational cohort study.

SETTING: Infertility center performing IVF.

PATIENT(S): Eight hundred women with infertility undergoing frozen ET.

INTERVENTION(S): Blastocysts cryopreserved on days 5, 6, and 7 after retrieval were thawed and transferred.

MAIN OUTCOME MEASURE(S): Ongoing PRs (pregnancy developing appropriately into the second trimester). Thaw survival, implantation rates, and clinical PRs were also calculated.

RESULT(S): A total of 1,406 embryos were thawed with a survival of 90.7% for day 5, 83.7% for day 6, and 78.7% for day 7. Implantation rates were 43.3%, 28.9%, and 28.9%, respectively. Ongoing PRs were 43.9%, 32.9%, and 26.7%, respectively.

CONCLUSION(S): Blastocysts cryopreserved on day 7 have a lower, but clinically important potential. Embryos that do not achieve blastocyst stage on day 6 should not be universally discarded, but should be observed in culture 1 more day as 27% may result in an ongoing pregnancy.