If you are going through in vitro fertilization (IVF), you may wonder whether you should have a day 3 or day 5 or 6 blastocyst embryo transfer. The ideal day of embryo transfer varies from woman to woman and from cycle to cycle. Therefore it can be hard to assuredly say whether day 3 or day 5 is best.

In this article we explain the science behind embryo transfer and why your doctor may choose to do a day 3 or day 5 transfer. 

The history of embryo transfer
In the early days of IVF, embryo transfer took place soon after fertilization. The procedure was often performed a day or two after the egg retrieval. In those days, IVF laboratory conditions were different than they are now. Embryos were not able to survive for long outside of the human body.

As years passed, laboratory culture conditions began to improve. Embryos were developing well in the laboratory for up to three days after egg retrieval. For many years, day 3 embryo transfers were the norm.

The blastocyst stage
Researchers soon discovered that certain nutrients were critical to aid in embryo development past day 3. Labs implemented new standards for embryo development, and embryos were able to survive in the lab for up to 5 or 6 days. At this point, after 5 to 6 days of development, the embryos had reached the blastocyst stage of development. According to the American Society for Reproductive Medicine, 35 to 50 percent of all fertilized oocytes will develop to this blastocyst stage.

Embryo development
Growing embryos to the blastocyst stage seemed to help identify embryos with better chances for implantation, but even good quality blastocyst embryos do not always result in a successful pregnancy. Embryos that look healthy and well developed by day 3 do not always have the best potential for implantation and a healthy pregnancy. IVF clinics understand that if an embryo did not develop well to the blastocyst stage in the laboratory, it most likely would not have resulted in a healthy pregnancy or birth.

Blastocyst culture
IVF clinics find that blastocyst culture is a reasonable tool to use to select embryos with the best potential in patients who have many good quality embryos. To best support the growth of embryos to the blastocyst stage, a clinic must have a good IVF lab with good culture conditions.

This is something to pay close attention to when you are choosing an IVF clinic. At your first visit, feel free to ask your clinic about their culture conditions and embryo transfer practices.

All about embryo transfer
Some patients may only have one or two embryos that develop normally after retrieval. For these patients, there may not be an advantage of continuing on to the blastocyst stage. In these cases, clinics can transfer the embryo(s) to the uterus on day 2 or 3. For these embryos, the human body may be the optimal place for the embryo to grow and develop after fertilization.

For most patients, clinics transfer fewer embryos at the blastocyst stage than at the day 3 stage of development. This practice usually results in the highest pregnancy rates, but reduces the risk of multiple pregnancies. This is due to better implantation rates per blastocyst embryo transferred. Embryos destined to become babies likely have the same chances of implantation whether transferred at day 3 or day 5.

Best day for embryo transfer
The question as to the best day for embryo transfer is still under debate. Here are some important recent scientific research findings about the topic:

  • Day 5 transfers may be ideal since in natural conception cycles, embryos typically implant on days 5 or 6 after ovulation. According to this theory, day 5 or 6 blastocyst embryo transfers may be preferable due to the ideal uterine environment conditions at this stage. Embryos transferred at the blastocyst stage often have a higher implantation rate (meaning that they are more likely to “stick” and develop into a healthy pregnancy).
  • Studies show a slightly higher rate of identical twins with blastocyst transfers. Identical twins put a pregnancy at higher risk for early delivery and other complications. However, elective single blastocyst transfer is shown to reduce the chances of conceiving twins.
  • Studies have also shown that patients with a good prognosis may experience higher live birth rates with blastocyst transfer on day 5 or 6. Those with a poor prognosis may be better suited for a day 3 embryo transfer.

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