When there are choices to make, some patients feel afraid that they are making the wrong one. The good news here is that there is no wrong choice. Day 5 and Day 3 transfers have the same success rates. However, here is the whole story:
In the early days of IVF, we put fresh embryos back on Day 2, because everyone was afraid that keeping them in the lab longer would be harmful. Everyone assumed the body is a better incubator. Then, without any change in lab conditions or media, people started doing day 3 transfers, and it proved to be safe. In those days, we still tended to put back several embryos to achieve the best success rates. As we all know, this led to a rather high and unacceptable multiple birth rate.
In the late nineties, a few labs began experimenting with culture media that could support embryo growth until Day 5 when they reach the so-called, “blastocyst” stage. Unlike the 6-8 cell embryos we see on day 3, blastocysts contain dozens of cells, and already has differentiated into cells that will become the embryo (inner-cell mass), surrounded by a ring of cells destined to become the placenta (trophoblasts or trophoectoderm). The first paper that reported the successful transfer of blastocysts merely showed that the same success rates are achieved using 3-day 3 embryos vs. 2-day 5 embryos. Obviously, the latter group had fewer triplets. They never claimed a better success rate; just a higher implantation rate.
Over the years, most labs have become proficient at growing embryos to day 5. Currently, PGD/PGS biopsies are performed on blastocysts. The main benefit of day 5 transfers is embryo selection. If a woman has not made enough embryos to select from, day 3 transfers are fine.
Some people have lingering concerns about the safety of keeping the embryos in the lab for 5 days. There is little evidence to support these concerns. The health benefits of avoiding multiple births would likely outweigh these worries. A couple of companies have developed “embryo scopes” that assess the growth of the embryos with time-lapse photography. They improve the prediction of which day 3 embryos would likely make it to blastocyst stage. Time and experience will tell if these innovations will replace day 5 culturing or PGS for embryo selection.