Suboptimal laboratory conditions can affect pregnancy outcome after embryo transfer on day 1 or 2 after insemination in vitro.

We have demonstrated that the replacement of pronuclear stage human embryos on the day after insemination in IVF-ET results in a pregnancy rate as good as, if not better than, the rate after ET on day 2. The pregnancy rate appears to be influenced by the culture environment used for the gametes. We would recommend that ET be undertaken as soon as possible after insemination on day 1 so that if suboptimal culture conditions are present, they do not have sufficient time to affect the viability of the embryos and reduce the subsequent chance of pregnancy after replacement. If culture conditions have been optimized there is no difference in pregnancy rate after embryo transfer on day 1 or day 2 and one can then replace embryos on either day and expect the pregnancy rate obtained in either case to be similar.