 An aromatase inhibitor blocks the conversion of androgens to estrogens in the ovarian follicles, peripheral tissues, and in the brain. This result in two things: a Fall in circulating and local estrogens and b Rise in intraovarian androgens. Fall in estrogen levels, releases the hypothalamopituitary axis from the negative feedback of estrogens. Thus, there is a surge in follicle stimulating hormone FSH release, which results in follicular growth. Since, the feedback mechanism is intact; normal follicular growth, selection of dominant follicle, and atresia of smaller growing follicle occurs; and thereby facilitating monofollicular growth and ovulation.Another mechanism of action of the aromatase inhibitors is by the increasing intraovarian androgens. This increases the follicular sensitivity to FSH. Recent data shows the role of androgens in early follicular developments. by augmenting FSH receptors and stimulating insulin-like growth factor IGF-I; FSH and IGF-I act synergistically to promote follicular growth. This pharmacodynamics of letrozole ensures improved endometrial thickness, cervical mucus, monofollicular, and better folliculogenesis. So , these factors may lead to higher pregnancy rates and greater likelihood of singleton pregnancy. 