Time to switch from co-culture to sequential defined media for transfer at the blastocyst stage.

It is current practice in in-vitro fertilization (IVF) to transfer embryos 48 h post-insemination at the 2–4 cell stage. However, it is now clear that this is a totally blind process. Firstly, embryos encounter an unsuitable environment, normally they would be in the oviduct. Secondly, motility of the uterus is related inversely to circulating progesterone. This problem of hypermotility of the uterus around ovulation is a well known phenomenon in large domestic animals where uterine transfer of morulae and blastocysts is the only option available. The fact that embryos block around genomic gene activation, owing to maternal (Janny and Ménézo, 1996), paternal (Ménézo and Dale, 1995) and cytogenetic problems (Benkhalifa et al., 1996), promotes the transfer of unreasonably high numbers of embryos with high rates of multiple pregnancies. In accordance with previous suggestions (Ménézo et al., 1992, 1997; Olivennes et al., 1994; Bavister and Boatman, 1997; Gardner and Lane, 1997), blastocyst transfer is highly recommended for the following indications.