Embryo transfer refers to a step in the process of assisted reproduction in which embryos are placed into the uterus of a female with the intent to establish a pregnancy.

Embryo transfer is a simple procedure that follows in vitro fertilization (IVF) and is often considered the simplest and final step of the in vitro fertilization process. The objective of embryo transfer is to facilitate conception following fertilization from the in vitro fertilization procedure.

When Does the Embryo Transfer Procedure Occur?

Embryos are generally transferred to the woman’s uterus at the 2-8 cell stage. Embryos may be transferred anytime between day 1 through day 6 after the retrieval of the egg, although it is usually between days 2-4. Some clinics are now allowing the embryo to reach blastocysts stage before transferring, which occurs around day 5.

Fresh or Frozen Thawed Embryos to Transfer?

Newborns from the fresh ET group had lower birthweight than the frozen-thawed ET group.

Embryo aneuploidy is likely the leading cause of implantation failure in IVF cycles. There is well-documented evidence of increasing maternal age directly correlating with an increase in embryonic aneuploidy rates. With recent advances in IVF (extended embryo culture, trophectoderm biopsy, and vitrification), along with the combination of new and advanced technology in preimplantation genetic screening (PGS), ongoing pregnancy rates (PRs) have improved with the selective transfer of euploid blastocysts. Preimplantation genetic screening is routine in some clinical IVF practices in the United States.

The two transfer strategies for euploid embryos currently in clinical practice are to use vitrified/warmed (“freeze-all”) or fresh embryos for the first ET. The freeze-all strategy involves cryopreservation of all embryos after biopsy, and then waiting for the PGS results of the whole cohort (day 5 and day 6 embryos) in preparation for a frozen ET. The fresh strategy involves biopsy of expanded blastocysts before 10 am on day 5 and cultured overnight to await PGS results for a fresh ET of euploid embryos before noon on day 6. In this scenario, slower growing embryos may be biopsied on day 6 and frozen for later use.

There are benefits and challenges to each approach. There is evidence that implantation and clinical ongoing PRs may be higher when transferring vitrified/warmed embryos in a non stimulated cycle compared with fresh transfer in a stimulated cycle. The incidence of low birthweight babies and preterm delivery has also been shown to be lower in pregnancies resulting from frozen transfers compared with fresh transfers.