Figure 32.7 Regimens of ovarian stimulation and hormone replacement used to synchronize the development of ovarian follicles in the oocyte donor and the endometrial cycle in the recipient. hCG, human chorionic gonadotropin; OCP, oral contraceptive pill; GnRH-a, gonadotropin releasing hormone antagonist; TVA, ultrasound-guided transvaginal aspiration of oocytes. (Adapted from Chang PL, Sauer MY. Assisted reproductive techniques. Stenchever MA, ed.; Atlas of clinical gynecology, Mishell DR, ed, Reproductive endocrinology. Vol. 3. Philadelphia, PA: Current Sciences Group, 1998, with permission.) used for fresh transfer and lesser quality embryos are frozen) (373,376). Frozen transfer outcomes are heavily dependent on the characteristics of the fresh cycle that generated the frozen embryos; excellent pregnancy rates are noted when the fresh cycle resulted in conception or when all the embryos were frozen from the fresh cycle (376).
Endometrial Preparation for Frozen Embryo Transfer When FET is combined with a recipient’s natural cycle, no exogenous treatment is given, and transfer is timed to spontaneous ovulation. In medicated FET cycles, estradiol supplementation begins in the early follicular phase and is continued for 13 to 15 days (373). Multiple estradiol preparations have been described for use in FET cycles, but none has been proven superior (377). Transvaginal ultrasound is used to assess endometrial thickness during estrogen therapy, and estrogen administration continues until an optimal thickness of greater than 8 mm is reached (373). Progesterone supplementation begins 48 to 72 hours prior to transfer when cleavage-stage embryos are used and 6 to 7 days prior to transfer when blastocysts will be thawed (373,377). Again, several progesterone preparations have been described for use in FET cycles, but none has been proven superior (discussed in the “Uterine Factors” section). GnRH agonists are commonly used during medicated cycles to prevent premature LH surges that might adversely affect endometrial maturation (377) (Fig. 32.7).
The production of hCG by the blastocyst can be detected as early as 7 days posttransfer, and serum quantitative hCG levels may be obtained 11 to 14 days following embryo transfer (373,378). A serum threshold of 200 mIU/mL of hCG measured 12 days after transfer is 92% and 80% predictive of ongoing pregnancies for day 3 and day 5 embryos, respectively, with levels normally rising approximately 40% per day. If normally rising hCG levels are detected, transvaginal ultrasound is planned at 6 to 7 weeks of gestation to determine the location of the pregnancy, the number of gestational sacs, and pregnancy viability (378).