There are two possible options for performing a frozen embryo transfer (FET): Natural-Cycle FET (NC-FET) and medicated FET.
Natural Cycle FET is available to women with regular ovulation and monthly menstrual cycles. In patients with predictable menstrual cycles, we can carefully monitor the cycle to determine the precise timing of ovulation. Alternatively, ovulation can be induced with the administration of an HCG injection. Once the precise date of ovulation is set, then the uterine lining should be receptive to embryo transfer 5 days later (for embryos frozen on day 5 in a previous IVF cycle). In this way, the embryos can be replaced at approximately the time when they would normally be arriving in the uterus.
One problem with Natural-Cycle FET is that the optimal time for implantation may fall at an unpredictable time during the laboratory work schedule, so some clinics choose not to offer NC-FET. In addition, NC-FET demands frequent patient monitoring around the time of ovulation. If a cycle is suboptimal in terms of the estrogen level and endometrial development, then the embryos should not be thawed and the cycle should be cancelled.
A medicated FET allows the couple to avoid some of the pitfalls associated with a NC-FET. In this type of FET, estrogen pills, shots, or patches are used to prepare the endometrium for embryo implantation. Three days prior to embryo transfer, the woman begins taking progesterone to modify the endometrial lining so that it will be receptive when the embryos are placed. Some clinics prescribe GnRH agonists (such as Lupron) to their patients the month prior to a medicated FET cycle so as to reduce the chances of spontaneous ovulation. The use of Lupron reduces the chances of cycle cancellation owing to unexpected ovulation to near zero.
The choice of estrogen supplementation varies somewhat between clinics. Some clinics prescribe oral estrogens; other clinics administer intramuscular estrogen shots twice a week, and still other clinics use transdermal estrogen patches twice a week. The choice of estrogen protocol is clinic-specific and should be discussed with your reproductive endocrinologist. Many clinics prescribe progesterone-in-oil injections to prepare the lining for embryo transfer. Women who have a progesterone allergy can use progesterone suppositories instead.
Pregnancy rates have not been shown to differ significantly between NC-FET and medicated FET. However, medicated FETs do allow for advance scheduling, which both patients and physicians find attractive. The advantages of NC-FET are its simplicity with no injections (besides the HCG trigger shot) and minimal additional medications with an overall cost savings compared with a medicated FET.
To learn more about frozen embryo transfers, head over to the FET category of our fertility blog.