Estrogen is the primary female sex hormone (in its active form, estradiol). It also refers to a family of estrogenic compounds that are produced by women's bodies during different stages of their lifecycle (estriol during pregnancy and estrone after menopause). Together with the other major family of female sex hormones, progestins, it promotes the development and maintenance of female characteristics of the body.
Greater exposure to estrogens over the course of women's ontology seems to have an effect on woman's eye and lip size, with higher estrogen equating to thicker, brighter colored lips as well as large eyes, resulting in a more neotenous, i.e. more child-like appearance. It promotes other female sex typical characteristics, such as women's generally higher body fat percentage and their higher level of subcutaneous fat (fat deposited under the skin), generally leading to a smooth and "curvy" appearance in women. It also seems to contribute to a female typical cognitive profile possibly depressing spatial intelligence and promoting certain types of verbal intelligence (such as greater verbal fluency).