The impact of embryo quality and quantity on implantation and the establishment of viable pregnancies.

The role of cleavage rates, number of embryos transferred, and some other variables in pregnancy outcome in 222 human embryo transfers was studied. Pregnancy rates were significantly higher in a group of 117 patients receiving at least one embryo that had reached the four-cell stage at 40 hr postinsemination (26% total pregnancies/transfer and 18% ongoing pregnancies/transfer) than in the 105 patients receiving embryos developing at a slower rate (7% total pregnancies/transfer and 2% ongoing pregnancies/transfer). Of the 23 ongoing or term pregnancies produced, 21 came from transfers of at least one embryo that had reached four-cell stage by 40 hr postinsemination. Pregnancy rates were unaffected in either the fast-cleaving or the slower-cleaving embryos by culturing in vitro for an additional 24 hr. The presence of anucleate cell fragments also had no effect on pregnancy rates. Pregnancy rates increased progressively with the transfer of more embryos per transfer. These results suggest that procedures to improve ovulation induction and in vitro embryo culture technique may better the success of in vitro fertilization by providing a high number of rapidly cleaving embryos for transfer.