After treatment with gonadotropin , clomiphene , or letrozole , clinical pregnancies occurred in @ % , @ % , and @ % of cycles , and live birth in @ % , @ % , and @ % , respectively ; pregnancy rates with letrozole were significantly lower than the rates with standard therapy ( gonadotropin or clomiphene ) ( P = @ ) or gonadotropin alone ( P < @ ) but not with clomiphene alone ( P = @ ) .