Natural vs stimulated cycle for frozen embryo transfer

This study compared modified natural cycle to stimulated cycle prior to the transfer of a frozen embryo. It found that birth rates were similar for both methods, although there was a trend toward increased early pregnancy loss with STC.

In vitro fertilization (IVF) is an infertility treatment which stimulates the ovaries to produce multiple follicles, and fertilizes the oocytes (eggs) in the laboratory. The embryo(s) can be transferred to the uterus in the same cycle, or they may be frozen to transfer in a future cycle. There are multiple reasons to use a frozen embryo transfer (FET), including having extra embryos or conducting genetic testing.

When FET is used, it is important that the lining of the uterus is prepared for an embryo to implant. After ovulation, high levels of the hormone progesterone prepare the lining. Natural cycle FET monitors the menstrual cycle and performs the transfer after ovulation. In contrast, stimulated cycle uses hormone injections to artificially stimulate follicle development and ovulation. It is unclear which method leads to better FET outcomes. It is also unclear which method is preferred by patients, and whether the modified natural cycle (MNC) requires more visits to the clinic.

This study included 119 patients who were undergoing their first or second FET. All patients had regular menstrual cycles. 59 patients were randomly assigned to MNC, and the other 60 patients had a stimulated cycle (STC).

Patients in the STC treatment group were given injections of recombinant follicle-stimulating hormone (rFSH) to stimulate follicle development. When a follicle was over 17 mm, they were given recombinant human chorionic gonadotropin (rHCG) to trigger ovulation. Patients in the MNC were not given any hormone injections. Both groups were monitored with ultrasound and blood tests. Progesterone levels were used to estimate the day of ovulation for both groups.

The STC group required significantly fewer visits to the fertility clinic prior to the FET (3.6 vs. 4.4) and a lower number of blood tests (2.7 vs 3.5). The number of FETs performed outside of business hours was similar between the two groups (22.6% – STC vs. 27.5% – MNC). Patients were given the FertiQol questionnaire to determine their quality of life during treatment. The two groups responded similarly.

30 patients had a positive pregnancy test. There was a slightly higher rate of positive pregnancy tests in the STC group (34.0%) compared to the MNC group (23.1%). However, the STC group had three miscarriages and two ectopic pregnancies. There were no early pregnancy losses in the MNC group. The two groups had similar rates of live birth (24.5% – STC vs. 23.1% – MNC).

This study found that MNC and STC led to similar live birth rates from FET. MNC required an average of one additional visit to the clinic, and the patient quality of life was similar for both methods.

This study would have required more patients to detect small differences in pregnancy rates. Also, the study did not compare the rates of pregnancy loss using statistics.