After treatment with gonadotropin , clomiphene , or letrozole , clinical pregnancies occurred in 35.5 % , 28.3 % , and 22.4 % of cycles , and live birth in 32.2 % , 23.3 % , and 18.7 % , respectively ; pregnancy rates with letrozole were significantly lower than the rates with standard therapy ( gonadotropin or clomiphene ) ( P = 0.003 ) or gonadotropin alone ( P < 0.001 ) but not with clomiphene alone ( P = 0.10 ) .