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What is the difference between a Day 3 and a Day 5 Embryo Transfer?

September 2, 2011

To first answer the question about the differences between a Day 3 and a Day 5 embryo transfer, we need a little reminder from our biology classes.

An embryo begins its growth as a single cell, and then divides every 12-24 hours. By Day 3, it is about 4-8 cells, and is referred to as a multicell embryo. During the early growth of the multicell embryo, all of the energy and chemicals required for cell division come from the mother’s egg. By Day 5, the embryo, now called a blastocyst, is about 70-100 cells. A blastocyst has differentiated and contains two different cell types. The first is called the inner cell mass, which develops into fetal tissue. The second is called the trophoblast or trophoectoderm, and these cells lead to part of the placenta. In order to form a blastocyst, an embryo activates it’s own genes through a process called genomic activation. Genomic activation allows the embryo to produce the energy required for further cell division, differentiation, and blastocyst formation. The ability of an embryo to use it’s own genes and make it’s own energy is not simple. Approximately one third of embryos are capable of successfully activating their genes and growing to the blastocyst stage. This is what makes a blastocyst so special!

When deciding which embryos to transfer, Dr. Hummel, Dr. Kettel and SDFC embryologists are interested in the “best” embryos of the group. The meaning of “best” is different for all patients, because all groups of embryos are unique. Sometimes, the best embryos show themselves right away, while others take a few days. This is why our doctors feel it is important to watch the embryos grow in the lab before making a determination on transfer day.

With a small number of dividing embryos, the best embryos are usually more evident after only a few days in the lab. It is easier to see which two (two will be out number for transfer for the sake of this article) embryos are appropriately dividing, or are the best. Here, it is appropriate to transfer them earlier, because it is known which embryos are the best at an earlier time. The advantage is that the uterine environment may contribute to embryo growth, successful blastocyst development, and implantation, and so our doctors recommend transferring the best embryos on Day 3.

With a larger number of embryos, many of them may be of similar quality after three days, and it is difficult to determine which ones are the best. In this case, it is beneficial to allow the embryos to sort themselves out, and see which two embryos are capable of forming blastocysts. Therefore, Dr. Hummel and Dr. Kettel would recommend a Day 5 embryo transfer. In this case, the advantage is that the physicians know that the embryos they transfer have successfully made blastocysts – a big deal!

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