The goal of birth control is to prevent pregnancy. For most hormonal methods, this is achieved by completely preventing the release of an egg from the ovary. Hormonal contraceptives introduce synthetic versions of estrogen and progestin to alter the reproductive cycle. The question of what “week” ovulation occurs is not applicable for most users because the process is actively shut down throughout the entire cycle.
The Primary Goal: Preventing Ovulation
Combined hormonal contraceptives, such as the pill, patch, or ring, work by tricking the body into thinking it is already pregnant. They contain synthetic forms of both estrogen and progestin, which create a continuous state of negative feedback on the brain’s hormonal control center. This constant feedback suppresses the release of gonadotropin-releasing hormone from the hypothalamus.
This suppression, in turn, prevents the pituitary gland from releasing the necessary amounts of Follicle-Stimulating Hormone (FSH) and Luteinizing Hormone (LH). FSH is responsible for maturing the ovarian follicles, and the LH surge is the specific trigger for ovulation. By keeping the levels of these hormones low and eliminating the mid-cycle LH surge, no dominant follicle develops or ruptures to release an egg.
Since combined hormones inhibit follicular development and egg release, ovulation cannot take place during the monthly cycle. The bleeding that occurs during the placebo week of a combined pill pack is not a true menstrual period but a withdrawal bleed caused by the temporary drop in synthetic hormones.
Exceptions: Birth Control Methods That Don’t Guarantee Ovulation Suppression
Some contraceptive methods rely on mechanisms other than the complete shutdown of ovulation. Progestin-Only Pills (POPs), often called minipills, primarily work by greatly thickening the cervical mucus, creating a barrier that sperm cannot penetrate. The synthetic progestin also thins the lining of the uterus, making it unreceptive to implantation should an egg be fertilized.
Ovulation suppression is a secondary, and often inconsistent, effect of the older, lower-dose POPs, which may only suppress egg release in approximately 40% to 70% of cycles. Newer formulations of POPs, such as those containing desogestrel, are more effective at suppressing ovulation, reaching rates closer to 97%. Other progestin-only methods, like the implant or injection, contain higher doses that reliably inhibit ovulation.
Non-hormonal methods, such as the Copper Intrauterine Device (IUD) and barrier methods like condoms, do not affect the body’s natural cycle. With these methods, the ovaries continue to release an egg every month, and ovulation occurs naturally, typically about two weeks before the next period. The Copper IUD prevents pregnancy by creating a toxic environment for sperm and eggs, while barrier methods physically block sperm.
Compliance and Interference: Increasing Ovulation Risk
Factors can undermine the contraceptive effect of suppressive methods and lead to breakthrough ovulation. Missing doses is the most common reason for failure, especially for POPs, which must be taken within a very narrow window of time—often within three hours—to maintain the necessary hormone concentration. The short half-life of the progestin means a delay quickly drops the hormone levels below the threshold needed for protection.
Certain medications interfere with the body’s ability to maintain suppressive hormone levels. Drugs that induce liver enzymes, such as the cytochrome P450 system, cause the body to metabolize the contraceptive hormones more quickly. This rapid clearance reduces the hormone concentration in the bloodstream, raising the risk that the pituitary gland will resume its normal function and trigger ovulation.
Interfering substances include the antibiotic rifampin, certain anti-seizure medications such as carbamazepine and phenytoin, and the herbal supplement St. John’s wort. Severe or persistent gastrointestinal issues, like vomiting or diarrhea, can prevent the proper absorption of hormones from oral pills, similarly reducing the effective dose. When hormone levels drop unexpectedly, the protective mechanism fails, and spontaneous ovulation can occur, increasing the risk of an unplanned pregnancy.