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 i n human in-vitro fertilization (IVF) and embryo transfer. It determines whi ch and how many embryos will be replaced, as pregnancy rates are directly r elated to number and quality of transferred embryos. This retrospective ana lysis included 1301 IVF and embryo transfer cycles to identify which factor s influenced embryo duality. Embryo quality did not correlate with maternal age, causes of infertility, ovarian stimulation parameters or embryo cohor t size. However, the mean score of transferred embryos was significantly hi gher for patients with more than five embryos compared to fewer than five e mbryos (P < 0.001), irrespective of maternal age. Patients tended to produc e a similar embryo quality from cycle to cycle, r = 0.33 (P < 0.001) for th e embryo cohort and r = 0.47 (P < 0.001) for the transferred embryos. Poor embryo morphology probably reflects oocytes with compromised development co mpetence 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.