Hormonal birth control (BC) methods, such as the pill, patch, and ring, use synthetic forms of the body’s natural reproductive hormones, estrogen and progestin. These synthetic hormones enter the bloodstream and alter the reproductive system’s communication signals. The ovaries contain a finite number of immature eggs, known collectively as the ovarian reserve, which is established before birth and cannot be replenished. A primary question is whether hormonal contraception halts the natural progression of egg use and loss, or if the process continues regardless of the medication.
The Natural Fate of Eggs Without Birth Control
Each month in a natural cycle, a selected group of small, immature egg-containing structures, called follicles, begins the process of maturation. This recruitment of a follicular cohort is the first step toward potential ovulation. Follicle-Stimulating Hormone (FSH) from the pituitary gland signals these follicles to grow and develop.
As this group of follicles matures, they compete with one another for dominance. Typically, only one follicle becomes dominant, reaching full maturity and preparing for release. The remaining follicles in that monthly cohort, which can number around 15 to 20, stop developing and are naturally eliminated.
This programmed loss of follicles, known as atresia, is a continuous process that occurs throughout a person’s reproductive life. Atresia is an irreversible process of degenerative cell death, accounting for the loss of approximately 99% of all follicles a person is born with. Only about 300 to 400 follicles will ever reach the point of ovulation during a person’s entire reproductive lifespan. The process of atresia constantly reduces the ovarian reserve, eliminating around 1,000 follicles every month before menopause.
Preventing Ovulation: The Primary Action of Hormonal Contraception
Hormonal contraception primarily works by suppressing the communication between the brain and the ovaries. The synthetic hormones in combined oral contraceptives, patches, and rings create a negative feedback loop, signaling to the pituitary gland that the body has sufficient hormone levels. This “false signal” suppresses the release of gonadotropin-releasing hormone (GnRH) from the hypothalamus, which reduces the output of the pituitary hormones, FSH and Luteinizing Hormone (LH).
FSH is the hormone responsible for initiating the growth of the follicular cohort each month. With the level of FSH suppressed by the synthetic hormones, the ovaries do not receive the necessary signal to begin the maturation process of the follicles. This suppression prevents the recruitment and growth of a dominant follicle.
The suppression of LH is equally important, as it prevents the final stage of the reproductive cycle: the LH surge. In a natural cycle, a rapid, sharp increase in LH is what triggers the fully mature follicle to rupture and release the egg, which is the definition of ovulation. By blocking the LH surge, hormonal contraceptives ensure that even if a follicle were to partially develop, the final trigger for egg release is absent.
By maintaining steady levels of synthetic hormones, the medication effectively puts the ovaries into a state of rest, preventing the monthly development and release of an egg. This mechanism is the primary way these medications achieve pregnancy prevention. Progestin-only methods also thicken cervical mucus and thin the uterine lining, offering additional layers of protection.
The Continued Aging of the Ovarian Reserve
While hormonal birth control successfully prevents ovulation, it does not stop the natural, continuous decline of the ovarian reserve. The process of atresia, the programmed death of follicles, is largely independent of the hormonal cycle. Therefore, the eggs that would have been lost naturally through atresia each month continue to be lost, even while a person is taking the medication.
Birth control does not “save” eggs or pause the aging process. The age-related decline in both the quantity and quality of eggs is determined almost entirely by the person’s chronological age, a timeline that hormonal contraception does not alter. The ovarian reserve will continue to decrease at its natural rate throughout the years of medication use.
Current research suggests that while a person is actively taking hormonal contraceptives, markers used to assess ovarian reserve, such as Anti-Müllerian Hormone (AMH) levels and Antral Follicle Count (AFC), may appear lower. Studies have shown AMH concentrations can be temporarily reduced by about 19% to 31% in users compared to non-users. This temporary suppression is thought to be a direct result of the medication suppressing ovarian function, not a permanent depletion of the egg supply. Once the medication is stopped, ovarian function and the corresponding hormone markers are expected to return to the level appropriate for the person’s age.