The marked effectiveness of the combined oral contraceptive pill, which contains a synthetic estrogen and a progestin, is related to its multiple antifertility actions. The primary effect is to suppress gonadotropins at the time of the midcycle LH surge, thus inhibiting ovulation. The prolonged progestational effect also causes thickening of the cervical mucus and atrophic (not hyperplastic) changes of the endometrium, thus impairing sperm penetrability and ovum implantation, respectively. Progestational agents in oral contraceptives work by a negative feedback mechanism to inhibit the secretion of LH and, as a result, prevent ovulation. They also cause decidualization and atrophy of the endometrium, thereby making implantation impossible. In addition, the cervical mucus, which at ovulation is thin and watery, is changed by the influence of progestational agents to a tenacious compound that severely limits sperm motility. Some evidence indicates that progestational agents may change ovum and sperm migration patterns within the reproductive system. Progestins do not prevent irregular bleeding. Estrogen in birth control pills enhances the negative feedback of the progestins and stabilizes the endometrium to prevent irregular menses. Oral contraceptives have no direct effect on oocyte maturation and do not cause uterotubal obstruction.