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. β-Human chorionic gonadotropin (β-hCG) was administrated when the leading follicular size was ≥ 18 mm. Endometrial thickness was measured on cycle day 8, β-hCG day, transfer day and 7 days after embryo transfer. Estradiol levels were determined on the day of β-hCG administration. RESULTS: Patients' ages and number of previous unsuccessful IVT 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 β-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 7days 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 β-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.