One of the common questions that patients ask about IVF is the difference between doing an embryo transfer on Day 3 or Day 5 following egg pick up. To answer this, we should know a little about the reproduction physiology. When a woman conceives naturally, fertilization occurs in her fallopian tubes. It takes about 3 to 4 days for the fertilized egg to travel through the fallopian tube to the endometrial cavity of the uterus ( mother’s womb). During the transit of the fertilized egg (called a zygote at that stage), the zygote divides into an embryo (by day 2-3) and then further divides to a morula (by day 4) prior to arriving/implanting in the uterus. So, by the time the pregnancy implants in the uterus, it's at a stage of development called a blastocyst on day 5.

Investigations on this field show that blastocyst transfer (day 5) during IVF treatments results in a significantly higher chance of pregnancy compared to an embryo transfer on day 3.

We have routinely been performing blastocyst culture and transfer for many years and observe significantly higher implantation and pregnancy rates as compared to what we see with day 3 embryos. First of all, it could be that longer incubation of embryos in the laboratory allows healthier embryos to grow, while those that are not healthy generally stop growing. This would increase the likelihood that better quality embryos would be chosen and transferred into the patient’s uterus, potentially increasing the chance for pregnancy. Another possible reason is that it more physiologically mimics that which occurs in nature during a spontaneous pregnancy. IVF labs which have poorer techniques and personnel, tend to favour day 3 transfers, as they may not be able to nurture the growing embryos for 5 days. However, if a very limited number of embryos are available on day 3 and no embryo selection is required, then the benefit of a day-5embryo transfer may be limited to the improved synchronization between embryo and endometrium. Because it is obvious that the best conditioned place for the embryos is the mother’s womb itself.

Finally, a clinic’s success with embryo cryopreservation following extended culture should also be carefully considered. We believe that extended culture to day 5 must go hand in hand with an excellent cryopreservation program in order to maximize patient success.