Frozen-Thawed Embryo Transfer

Frozen-thawed embryo transfer is the transfer of embryos to the uterus after having been previously cryopreserved and thawed. This is an important procedure in treating infertility, and has generally positive outcomes when compared with fresh embryo transfers.

Cryopreservation is often required for patients who are not good candidates for fresh embryo transfer, such as women who have a high risk for ovarian hyperstimulation syndrome, or those requiring a quarantine period for embryos or even unfertilized oocytes.

After the initial consultation, screening, and other tests have been completed; the woman receives gonadotropin-releasing hormone or follicle-stimulating hormone to stimulate her ovaries. When the oocytes are ready, transvaginal ultrasound-guided oocyte retrieval is carried out. Fertilization is then achieved usually by intracytoplasmic sperm injection (ICSI).

The freezing protocol for embryo cryopreservation can be either slow programmable freezing or vitrification. When the patient approaches the appropriate time in her menstrual cycle, her uterus is prepared for implantation of the embryo. Prior to the transfer procedure, the recipient woman is given estrogen preparations two weeks in advance, then a combination of estrogen and progesterone to develop a uterine lining that is favorable for implantation.

Embryos are transferred three days after the egg is fertilized in-vitro, and are usually comprised of eight cells. However, they may be transferred after three more days, allowing them to develop further into blastocysts.

During the procedure, a speculum is inserted in the vagina to visualize the cervix. Then a soft transfer catheter containing one or two embryos is inserted through the cervical canal and advanced into the uterine cavity. This procedure is often facilitated with the use of ultrasound guidance to ensure correct placement, about 1-2 cm from the uterine fundus. Once the catheter is properly placed, the embryos are expelled from the catheter and deposited into the uterus. Finally, the embryologist inspects the catheter for any embryos that have been retained. Anesthesia is not required for this procedure.

Studies have shown that single-embryo transfer has better outcomes in terms of gestational age at delivery, birth weight, and mode of delivery (i.e. caesarean or vaginal delivery). Not surprisingly, twinning rates are lower, which mitigates the moral and ethical challenges associated with multiple gestation.

A clinical study in 2011 showed a survival rate of almost 80% for the embryos after thawing. Women who showed an endometrial thickness greater than or equal to 8 mm on the day of embryo transfer were also seen to have a higher pregnancy success rate.

Other factors that facilitate pregnancy for women undergoing frozen-thawed embryo transfer include the use of gonadotropin releasing hormone agonists and treatment using a combination of follicle-stimulating hormone and luteinizing hormone. Women with a positive fresh-cycle pregnancy test have also been shown to have a higher success rate for pregnancy, as well as those at a maternal age younger than 35 years.