Cleavage stage of human embryos two days after fertilization in vitro and their developmental ability after transfer into the uterus.

In 126 selected patients treated with clomiphene and human menopausal gonadotrophin, 154 embryos were transferred into the uterus 41-50 h after in-vitro insemination. Fifty per cent of embryos implanted. The rate of cleavage was inversely related to the success of embryo transfer; implantation failures occurred for 69 and 46% of embryos with more than or less than four cells, respectively (p less than 0.05). Cleavage rate was not related to the viscosity of the cumulus cell mass, serum supplementation of culture medium, nor to the age or genetic factors of the woman. There was a trend towards smaller follicles, and a significant (p less than 0.05) tendency for patients developing numerous follicles to yield faster-cleaving embryos. These relationships were not found in 50 other embryos obtained from patients treated with pure FSH.