Letrozole at the Crossroads of Efficacy and Fetal Safety in Ovulation Induction: A Narrative Review.

Letrozole, a third-generation aromatase inhibitor initially developed for breast cancer, has become the preferred first-line agent for ovulation induction (OI), particularly in women with polycystic ovary syndrome (PCOS). This narrative review critically evaluates the efficacy, safety, and clinical applications of letrozole across diverse infertility contexts. Compared to clomiphene citrate, letrozole is associated with higher ovulation and live birth rates, a lower risk of multiple gestation, and a more favorable endometrial environment. Its pharmacokinetics-marked by transient estrogen suppression and a short half-life-limit embryonic exposure, supporting its favorable safety profile. Emerging data from large, randomized trials and meta-analyses demonstrate no increase in congenital anomalies, miscarriage, or adverse perinatal outcomes in letrozole-conceived pregnancies. Moreover, maternal side effects are generally mild, and the risk of ovarian hyperstimulation syndrome is low. Letrozole has also shown utility in mild stimulation protocols, fertility preservation for estrogen-sensitive malignancies, and clomiphene-resistant PCOS. Key clinical strategies-such as early-cycle initiation, lowest effective dosing, and individualized monitoring-optimize therapeutic outcomes while minimizing potential risks. While long-term offspring data remain limited and mechanistic concerns persist, current evidence robustly supports letrozole as a safe and effective option for OI, balancing reproductive success with maternal-fetal safety across a range of infertility indications.