Aim: The aim of this study was to examine the effects of estradiol valerate (EV) on the thickness of clomiphene citrate (CC)-stimulated endometrium.

Material and Methods: Thirty-four normal ovulatory women were randomized double-blindly into two groups to receive CC 100 mg/day on day 2–6 of the treatment cycle, and either vitamin B (placebo) or EV 6 mg/day on day 10–14 of the cycle. The endometrial thickness, endometrial pattern, numbers of mature follicles, and maximal diameters of preovulatory follicles were evaluated by transvaginal sonographic examination.

Results: Thirty women completed both treatment cycles. Two other participants dropped out during the treatment due to side-effects (headache). The average endometrial thickness of the group treated with CC + placebo became slightly thinner when compared to the thickness at the baseline (9.04 vs 9.52 mm; P = 0.24). The CC + placebo and the CC + EV resulted in similar endometrial pattern, ovulation day, numbers of mature follicles, and sizes of the leading follicles before ovulation. However, an addition of EV into the CC cycle significantly increased the average endometrial thickness (10.7 mm vs 9.04 mm; P < 0.001).

Conclusions: We concluded that the addition of 6 mg/day EV following the CC treatment can prevent the endometrial thinning without perturbing folliculogenesis and ovulation.