Modified natural frozen embryo transfers seem to give us the highest rates of success but more data is needed.

It seems that use of letrozole (Femara) during modified natural embryo transferers results in lower pregnancy rates.

It could be argued that this is due to lower embryo quality in patients with PCOS. Out of 42 single euploid embryo transfers in modified natural cycles stimulated with gonadotropins we have had 28 (67%, 95% CI 50-80%) fetal heartbeats (1/1/2023 through 5/31/25). This suggests that embryo quality in women with PCOS is fine and that gonadotropins are the preferred option for ovation stimulation in women with PCOS.

By October 2024 our clinic had mostly moved away from using oral medications such as clomid, femara, and tamoxifen during frozen embryo transfer cycles. For this reason our dataset on FET for non PGT tested embryos, (which begins in October 2024 and currently goes through June 2025) contains few data points for this category of FET protocols. In this dataset the average patient age was 31 years old. 59% of embryos were frozen on day 5, 38% on day 6, and 3% on day 7.

The expected rate of fetal heartbeat per embryo is taken from a published reference standard on live birth rate based on embryo morphology and day of blastocyst formation (1). The expected fetal heartbeat rate per embryo is the expected live birth rate divided by 0.93 (this is a standard conversion we commonly use). For example a day 5 blastocyst from someone 33 years old at retrieval has an expected live birth rate of 40% and an expected rate of fetal heartbeat of 43%.

We need to continue to collect data to determine if the increase in fetal heartbeat rate for modified natural FETs without femara is statistically significant.

Expected FHT per embryo is based on SART data live birth rates for female age and embryo day (day5, day 6, or day 7).

Awadalla MS, Ho JR, McGinnis LK, Ahmady A, Cortessis VK, Paulson RJ. Embryo morphology and live birth in the United States. F&S Reports. 2022 Jun 1;3(2):131-7.

This research has been approved by the Christ Hospital IRB (protocol #25-030).