ABSTRACT
Since the beginning of in vitro fertilization (IVF), it has been unclear on what day after fertilization embryo transfer should best take place. In 1984, Edwards and colleagues performed a controlled trial comparing day-2 with day-3 transfers, and concluded that day-3 transfer led to an increased (although not statistically significant) pregnancy rate, when compared to day-2 transfer1. On the other hand, they observed significantly higher abortion rates after day-3 than after day-2 transfers, and this may have been the reason why, over the years, most people continued to transfer embryos on the second day after fertilization. In the 1990s, however, many laboratories shifted to day-3 transfers, although there was no conclusive evidence supporting this decision. Surprisingly, only a very low number of studies have been performed to compare day-2 with day-3 transfer in IVF, in contrast to the substantial number of trials comparing day-2 and day-3 (or early cleaving stage) embryo transfers with blastocyst transfers. Owing to the very high ongoing pregnancy rates obtained by some authors in the 1990s after the transfer of more than one (two and often more than two) day-5 embryos, usually in selected groups of patients, blastocyst culture and transfer in many centers has become the unquestioned default procedure.
