Does Increasing Estrogen Dose During Frozen Embryo Transfer Affect Pregnancy Rate?

Objective: To assess the effect of increasing estrogen doses during hormone therapy frozen embryo transfer (HT-FET) cycles on endometrial thickness and success rates compared to patients who received fixed estrogen dose.

Materials And Methods: A retrospective study from a university-based fertility clinic during the years 2008-2021. We compared two groups: the fixed-dose group (i.e., received 6 mg estradiol dose daily until embryo transfer) and the increased-dose group (i.e., the initial estradiol dose was 6 mg daily, and was increased during the cycle).

Primary Outcome: clinical pregnancy rate.

Results: The study included 5452 cycles of HT-FET: 4774 cycles in the fixed-dose group and 678 cycles in the increased-dose group. Ultrasound scan on days 2-3 of the cycle showed endometrial thickness slightly different between the two groups (4.2 mm in the fixed-dose and 4.0 mm in the increased-dose group, P = 0.003). The total estrogen dose was higher, and the treatment duration was longer in the increased than the fixed-dose group (122 mg vs. 66 mg and 17 days vs. 11 days, respectively; P < 0.001). The last ultrasound scan done before the addition of progesterone showed that the endometrial thickness was significantly thicker in the fixed than the increased-dose group (9.5 mm vs. 8.3 mm; P < 0.001). The clinical pregnancy rates were 35.8% in the increased-group vs. 34.1% in the fixed-dose group; P = 0.401.

Conclusions: The increased-dose group had thinner endometrium despite the higher doses of estrogen and longer treatment duration than the fixed-dose group. However, the pregnancy rates were similar between the two groups.