Birth control pills prevent pregnancy through several mechanisms, mainly by stopping ovulation. If no egg is released, there is nothing to be fertilized by sperm, and the woman cannot get pregnant. Most birth control pills contain synthetic forms of two female hormones: estrogen and progestin. These synthetic hormones stabilize a woman's natural hormone levels, and prevent estrogen from peaking mid-cycle. Without the estrogen bump, the pituitary gland does not release other hormones that normally cause the ovaries to release mature eggs.Specifically, synthetic estrogen in the pill works to:Stop the pituitary gland from producing follicle stimulating hormone (FSH) and luteinizing hormone (LH) in order to prevent ovulation.Support the uterine lining (endometrium) to prevent breakthrough bleeding mid-cycle.Meanwhile, synthetic progestin works to:Stop the pituitary gland from producing LH in order to prevent egg release.Make the uterine lining inhospitable to a fertilized egg.Partially limit the sperm's ability to fertilize the egg.Thicken the cervical mucus to hinder sperm movement (although this effect may not be key to preventing pregnancy).There are two kinds of hormonal birth control pills: (1) the combination pill which contains estrogen and progestin and (2) the progestin-only pill (known as the minipill). Combo pills are significantly more effective than progestin-only pills and have the added benefit of less breakthrough bleeding. However, some women cannot tolerate estrogen and prefer the progestin-only pill. Both types of pills are available in several different brands, each of which have slightly different blends of hormones.