Objective: To compare endometrial thickness after three consecutive cycles of ovulation induction with clomiphene citrate versus letrozole.

Methods: Eighty-four women with normal menstrual interval who attended the university infertility clinic from June 2016 to March 2017 were eligible for the study. After the endometrial thickness of baseline cycle was recorded, all participants were randomized into two groups of clomiphene citrate or letrozole treatment for three consecutive cycles. Endometrial thickness and estradiol level were measured when at least one follicle reached 17 mm in diameter. The differences in endometrial thickness relative to baseline of the two groups were compared.

Results: A total of 62 patients completed three cycles of ovarian stimulation. Both drugs resulted in significantly thinner endometrium compared with the baseline cycle thickness. The mean endometrial thickness was significantly decreased in the clomiphene citrate group compared with letrozole group (7.46 ± 1.71 vs 8.88 ± 2.34 mm, p = 0.029). Estradiol level on the day of induced ovulation was significantly higher in the clomiphene citrate group than in the letrozole group (706.0 (207.9, 2209.0) vs 168.7 (30.0, 401.8), p < 0.001). The number of the follicles reaching 17 mm on the day of induced ovulation was higher in the clomiphene citrate group (1.9 ± 0.8 vs 1.4 ± 0.5, p = 0.002).

Conclusion: Letrozole had less effect on endometrium thinning after three consecutive cycles of induced ovulation compared with clomiphene citrate.