Purpose: To compare the incidence of monozygotic twins following blastocyst versus day-3 embryo transfer (ET).
Methods: A retrospective analysis of the outcome of assisted reproductive technology (ART) cycles utilizing blastocyst ET during 1999-2000 was compared to a similar group of patients undergoing day-3 ET during 1997-1998.
Results: Blastocyst ET was used in 75 cycles with 2.0 +/- 2 embryos transferred. The comparison group consisted of 90 cycles with day-3 ET and 3.0 +/- 2 embryos transferred.
Conclusions: High pregnancy rates are maintained with blastocyst ET even though fewer embryos are transferred. The rate of monozygotic twins is higher with blastocyst ET than with day-3 ET. This increase may partially negate the benefit of reduced high-order multiple gestations attributed to blastocyst ET.