Scientific evidence seems to indicate that it is better to transfer embryos at the blastocyst stage, as they will achieve higher gestation rates than embryos at day 3. This increase in implantation rates is due, in part, to a better synchronization between embryo and endometrium, but above all, to a better selection of embryos. Not all day 3 embryos will make it to the blastocyst stage, so if we can discard these embryos by long culture, we will be avoiding useless transfers.
The first studies that were performed on day 3 and 5 embryo freezing seemed to find worse results with embryos at the blastocyst stage. This was because the freezing technique used was slow freezing, which decreased embryo survival to thawing, especially for blastocyst stage embryos. Nowadays, the technique used is vitrification, with embryo survival close to 100%, both for embryos at day 3 and embryos at day 5.
Thus, seeing that blastocysts will give us higher gestation rates than day 3 embryos and that they have a very high survival rate to freezing (using vitrification), it seems clear to deduce that it would be better to freeze day 5 embryos.
It is true that studies have failed to find significant differences in cumulative gestation rate by freezing and transferring embryos at day 3 or 5 of development. What has been shown is that by freezing the embryos at day 5 a smaller number of embryos are frozen and the number of transfers necessary to achieve pregnancy will also be lower.
