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Combined oral contraceptives primarily function by preventing pregnancy primarily through the suppression of ovulation. The formulation of combined oral contraceptives includes both estrogen and progestin, which work together to inhibit the hypothalamus and pituitary gland's secretion of the hormones FSH (Follicle-Stimulating Hormone) and LH (Luteinizing Hormone).

When FSH and LH levels are decreased, this prevents the ovarian follicles from maturing and inhibits ovulation, meaning that no egg is released for fertilization. Without ovulation, the possibility of pregnancy is significantly diminished. This mechanism is central to how these contraceptives are effective.

In addition to ovulation suppression, these contraceptives also have secondary mechanisms — such as thickening cervical mucus, making it more difficult for sperm to enter the uterus, and altering the uterine lining, which can reduce the likelihood of implantation if fertilization were to occur. However, the primary and most critical way these medications prevent pregnancy is through the inhibition of FSH and LH, thereby stopping ovulation.