What Is Blastocyst?

What is Blastocyst (Day 5) Embryo Transfer?

A blastocyst results from an egg that has been fertilized and grown in the A.R.T. laboratory for 5 or 6 days. It contains between 80 to 150 cells and has three distinct physical features (Fig. 1). The inner cell mass (ICM) is the group of cells that eventually gives rise to the fetus, the trophectoderm (TE) cells contribute to the placenta and the the fluid filled cavity know as the blastocoel (BC). During a natural conception, the embryo develops into a blastocyst as it is leaving the fallopian tubes and entering into the uterus. In order for any embryo (A.R.T. or natural) to implant into the uterus and achieve a pregnancy it must develop to the blastocyst stage of development. Thus a blastocyst transfer occurs when an embryo that has been growing in the lab for 5 or 6 days and reaches the blastocyst stage of development is returned to the uterus in attempt at pregnancy. Typically, in patient’s less than 37 years old no more than two blastocysts are transferred back to the patient’s uterus, patient’s between 37 and 40 years old 2 or 3 and in patient’s > 40 years old generally 3 blastocysts.

One of the main benefits of blastocyst transfer seems to be an increased chance of implantation and thus achieving a pregnancy. This may very likely be a result of the effect that extended culture in the laboratory has on the embryos or the fact that the embryo is placed back into the uterus at a more physiologically appropriate stage. It is also very likely that in general it is younger patients who have more retrieved eggs, thus more embryos available to grow in extended culture to the blastocyst stage and in typically have an increased chance of pregnancy compared to older IVF patient’s regardless of the day of embryo transfer. It is important to keep in mind that not all eggs that fertilize will end up growing for 5 days in the laboratory, typically it is expected that between 30-50% of high quality day 3 (6-8 cell embryos) embryos make it to the blastocyst stage of growth in the A.R.T. laboratory. Therefore, the decision to perform a day 3 or 5 embryo transfer is often simply based on the number of high quality embryos that a patient has growing in the laboratory, those with 5 or more are more likely to have a day 5 transfer. Each patient’s cycle is different and we carefully consider what is the best for you so you may have the best chance of becoming pregnant.