In the past in IVF programmes embryos were transferred on the 3rd day after egg collection. When many good quality embryos are available on day 3 it can be difficult to decide which are the best ones to put back and in these circumstances we can now give couples the possibility of opting for blastocyst transfer. The embryos are allowed to grow in the laboratory until they are 5 days old. The appearance of the embryo changes so instead of a group of 6 to 8 cells as seen on day 3, it accumulates fluid in the centre of the embryo and those cells which are going to form the baby (the inner cell mass) fuse together at the side of the embryo. The remaining cells go towards forming the placenta. The blastocyst is a much more advanced structure than the 3 day old embryo and the real advantage of transferring blastocysts is the high live birth rate associated with blastocyst transfer.

Although many couples wish to undergo blastocyst transfer it can only be used in those patients who have several high quality embryos on day 3 after egg collection and so although we can take a decision in principle to perform transfer on day 5 we can only take the final decision on day 3. For women who wish to get pregnant but are keen to avoid a twin pregnancy, a single embryo transfer using a blastocyst will give the them the highest chance.