Blastocyst Embryo Transfer

Transfer of embryos on the 5th day after egg retrieval (also known as blastocyst transfer) has been reported in some studies to result in a higher pregnancy rate than transfer on the 3rd day after retrieval. Not all embryos, however, will survive in the laboratory to the blastocyst stage. On average, approximately 30-40% of the eggs that initially fertilize will develop to a blastocyst stage embryo. There are several factors that may explain the reported higher implantation rates with day 5 transfers. Transfer of a blastocyst in the uterine cavity is more physiologic than transfer of a day 3 embryo. In spontaneous conceptions a blastocyst arrives in the uterine cavity approximately 5 days after fertilization. Transfer of a blastocyst in an IVF cycle therefore more closely mimics the natural state of events. Allowing the embryos to develop to the blastocyst stage in the laboratory may also help select the healthier embryos that have a greater potential to produce a pregnancy. It has also been proposed that women whose embryos develop to the blastocyst stage have a better prognosis for pregnancy regardless of the day of transfer and these women may do just as well with a day 3 transfer. The question of whether a day 5 transfer is better than a day 3 transfer remains controversial.

Despite the above controversy, it is our policy to perform day 5 transfers whenever possible. We will look at the embryo quality and number on day 3 and decide whether or not to transfer on day 3 or keep the embryos in culture until day 5. Because of the higher implantation rates of day 5 embryos, we usually transfer only two blastocysts, and occasionally only one. The risk of twins with a blastocyst transfer is still significant. There is also an increased risk of identical (monozygotic) twins in pregnancies resulting from a day 5 transfer. The reason for this is not clear at the present time. Identical twins are at increased risk for obstetrical problems compared to non-identical (fraternal) twins.