Selection of good embryos for transfer depends on embryo cohort size: implications for the 'mild ovarian stimulation' debate.

Embryo quality evaluated by the embryo morphology is a critical parameter in human in-vitro fertilization (IVF) and embryo transfer. It determines which and how many embryos will be replaced, as pregnancy rates are directly related to number and quality of transferred embryos. This retrospective analysis included 1301 IVF and embryo transfer cycles to identify which factors influenced embryo quality. Embryo quality did not correlate with maternal age, causes of infertility, ovarian stimulation parameters or embryo cohort size. However, the mean score of transferred embryos was significantly higher for patients with more than five embryos compared to fewer than five embryos (P < 0.001), irrespective of maternal age. Patients tended to produce a similar embryo quality from cycle to cycle, r = 0.33 (P < 0.001) for the embryo cohort and r= 0.47 (P < 0.001) for the transferred embryos. Poor embryo morphology probably reflects oocytes with compromised development competence and could be an independent factor of infertility. Furthermore, a large embryo cohort was the main factor increasing the chances of at least one good embryo in the cohort.