Embryo (Blastocyst) Transfer

After embryos are created in the IVF process, a selected number of specific embryos are transferred from the laboratory into the mother’s uterus. For a successful pregnancy and healthy baby the embryo or embryos must implant on the uterine wall. The term “blastocyst transfer” refers to performing this procedure at a specific time in the embryo’s development.

The question of when an embryo is transferred is important to the infertile couple’s IVF outcome. Historically, embryo transfer (ET) was performed on day three after fertilization. However, embryos are considered to be at the blastocyst stage of development immediately prior to hatching, around five days after egg retrieval. There is increasing evidence that waiting until that stage of development to transfer results in higher implantation rates per embryo.

Learn more about blastocyst transfer

Reasons for Higher Success Rates

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

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

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.

Also, 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.


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 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 six 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 five, day six blastocysts will be frozen for later use.