Embryo Transfer and Blastocyst Stage

What is an embryo transfer?

An embryo transfer refers to the part of the in vitro fertilization (IVF) process in which a doctor – utilizing ultrasound equipment and a pipette  – places an embryo or embryos into a woman’s uterus. The embryo transfer process only takes a few minutes and involves a short recovery time.

For a successful pregnancy and healthy baby, the embryo(s) must implant on the uterine wall. The term “blastocyst transfer” refers to performing this procedure at a specific time in the embryo’s development (around five days), which has been shown to have higher rates of implementation.

Additionally, the specific number of embryos transferred also has an effect on the chances of a healthy pregnancy. An elective single embryo transfer (eSET) is sometimes recommended to reduce the risk of a multiple pregnancy (twins or more), due to increased health risks to the mother and child.

Unused embryos may be frozen and implanted at a later date.

When does an embryo transfer occur during IVF?

Embryos develop through multiple stages, often described by the number of days since the egg was retrieved from the woman’s follicle.

The question of when an embryo is transferred into a women’s uterus is important to the infertile couple’s IVF outcome. Historically, embryo transfer (ET) was performed on day three after fertilization.

However, abundant evidence shows that transferring embryos at the blastocyst stage (the point in development just prior to hatching and implantation) into the uterus results in higher implantation rates per embryo. The blastocyst stage occurs around five days after fertilization.

Who should consider a blastocyst embryo transfer?

Blastocyst transfer may be of particular benefit for patients who:

  • Develop many good quality embryos
  • Wish to limit their risk of multiple pregnancy
  • Have past failed pregnancy with day 3 embryo transfer
  • Have poor quality embryos (less than 5 cell) on day 3.

Blastocyst transfers are routinely anticipated in patients having three or more good quality embryos on the third day after fertilization.

Medical history, maternal age and the number and quality of embryos created in the IVF cycle also play a part in whether or not a blastocyst embryo transfer is the best route for a particular patient.

Learn more about blastocyst transfer

Reasons for higher success rates

Transferring embryos to the uterus five days post-fertilization results in a uterine environment for the embryo that more closely resembles a healthy natural conception.

It is also easier for embryologists to choose healthier embryos for transfer when they are further along in development.

In effect, blastocyst transfer allows for the transfer of fewer embryos with a higher potential for implantation. This is an especially important point for infertility patients who want to take every possible step toward having a single healthy baby rather than a multiple pregnancy, which can cause serious complications and health risks to both mother and child.

In addition, patients who are also using preimplantation genetic diagnosis (PGD) will be using blastocyst transfer. Embryos that are biopsied in the PGD process are grown to the blastocyst stage before the biopsy.

Recent studies indicate that biopsy on the third day may result in a 15 percent reduction in pregnancy rate.

Fresh vs. frozen embryo transfers

Embryos resulting from IVF treatment are either transferred “fresh” or “frozen.”

A fresh transfer cycle refers to eggs that are removed, fertilized and cultured for several days in the lab and transferred into the uterus during one menstrual cycle.

A frozen embryo transfer cycle refers to quality embryos resulting from a previous egg retrieval cycle which are frozen for later use. These embryos can be thawed and used during the following menstrual cycle or years in the future.

Are “freeze all” embryo transfer cycles the best choice? Dr. Hinckley discusses in her blog.

Embryo transfer risks

There is a risk that none of the embryos will develop to blastocyst stage. On average, about half of fertilized eggs achieve the blastocyst stage. When there are no viable embryos to transfer, the distressing news might be tempered with the knowledge that the non-developing embryos would likely not have resulted in pregnancy if they had been transferred earlier.


Day 6 embryos

At RSC Bay Area, we have clear data showing that embryos take six days to become blastocysts and do better when transferred in a frozen cycle than on day 6 in a fresh cycle. We believe this is due to improved endometrial synchronization. For this reason, if there are no blastocysts to transfer on day 5, day 6 blastocysts will be frozen for later use.

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