Clomiphene citrate vs. letrozole for cryopreserved-thawed embryo transfer: a randomized, controlled trial.

Objective: To assess the endometrial thickness, hormonal status and pregnancy rates with clomiphene citrate (CC) vs. letrozole in frozen-thawed embryo transfer protocols.
Study Design: Nineteen patients who had previously undergone in vitro fertilization (IVF) with embryo cryopreservation were prospectively and randomly enrolled in a randomized, controlled trial. Nine patients were treated with CC, 100 mg per day, from day 3 to day 7 of the cycle, and 10 patients were treated with letrozole, 2.5 mg per day, on the same cycle days. beta-Human chorionic gonadotropin (beta-hCG) was administrated when the leading follicular size was > or = 18 mm. Endometrial thickness was measured on cycle day 8, beta-hCG day, transfer day and 7 days after embryo transfer. Estradiol levels were determined on the day of beta-hCG administration.
Results: Patients' ages and number of previous unsuccessful IVF cycles were similar between the 2 groups. Endometrial thickness was significantly higher in the letrozole group as compared to the CC group on the day of beta-hCG administration (9.1 +/- 3.6 vs. 6.9 +/- 1.2 mm, p<0.05), on the day of embryo transfer (10 +/- 1.7 vs. 7.6 +/- 1.4 mm, p<0.05) and 7 days after embryo transfer (12.2 +/- 0.4 vs. 9.0 +/- 3.0 mm, p<0.05). Estradiol levels in the letrozole group were significantly lower than in the CC group on the beta-hCG day (231 +/- 132 vs. 515 +/- 363 pg/L, p<0.05). Pregnancy was not achieved in either group.
Conclusion: Endometrial thickness is improved by letrozole as compared to CC in frozen-thawed embryo transfer cycles. Further investigation is needed to assess pregnancy and implantation rates.