Letrozole versus clomiphene citrate for unexplained infertility: a systematic review and meta-analysis

The objective of this study is to investigate and compare the effectiveness of letrozole and clomiphene citrate for improving fertility outcomes, including pregnancy rate, miscarriage rate, multiple pregnancy rate, and incidence rate of adverse events, number of dominant follicles, endometrial thickness at hCG day and serum E2 on hCG day. MEDLINE, EMBASE, CENTRAL, CNKI and CBMdisc databases were searched up to March 2013. Randomized controlled trials comparing letrozole with clomiphene in women with unexplained infertility were included. Pooled relative risk, mean difference and 95% confidence intervals were calculated. We found that there are no differences in pregnancy, miscarriage and multiple pregnancy rates, incidence rate of adverse events, number of dominant follicles (>18 mm) and endometrial thickness at hCG day in women with unexplained infertility between letrozole and clomiphene regimens. The mean (±standard deviation) concentration of serum E2 on hCG day was lower in those treated with letrozole than those with clomiphene. The subgroup of 2.5 mg letrozole displayed a statistically significant higher rate of clinical pregnancy as compared with 100 mg of clomiphene. The results of this study conclude that letrozole is as effective as clomiphene in women with unexplained infertility. Letrozole at a dose of 2.5 mg seems more effective. Further high-quality studies assessing the possible effectiveness of letrozole in selected groups of patients are warranted.