Hormonal Changes
According to Swathmore College's Department of Women's Studies, birth control pills typically contain a mixture of estrogen and progesterone. The estrogen works to inhibit the production of follicle stimulating hormone and luteinizing hormone, both of which are necessary for ovulation. The progesterone also blocks the production of luteinizing hormone and thins the lining of the uterus to make implantation of a fertilized egg (should one occur) be more difficult. As a result, stopping birth control causes the levels of luteinizing and follicle stimulating hormones to return to normal, which allows them to begin the typical rise and fall pattern that regulates the menstrual cycle.
Bleeding
According to the Mayo Clinics Birth Control Pill FAQ, most women experience some mild vaginal bleeding if they stop taking the pill (also known as spotting). This is the result of hormone levels changing as the body adjusts to no longer having a constant amount of estrogen. The uterine lining also begins to thicken as the progesterone leaves the body, which may cause it to prematurely slough off, which can add to the bleeding. However, this spotting is only temporary and is not considered to be serious, although it can delay the arrival of the next menstrual period. This is true whether you finish a pack of birth control pills before quitting or if you stop taking them mid-pack.
Fertility
The Mayo Clinic also notes that some women ovulate 2 weeks after they stop taking birth control pills. This means that during this time it is possible to get pregnant. Because stopping birth control pills can disrupt your menstrual cycle, some women prefer to wait a few months before trying to become pregnant so their menstrual cycle has time to restart. This allows them to pinpoint when they ovulated, which makes it easier to know when the baby is due. Some women do not have their period for many months after stopping the pill, however. This is called post-pill amenorrhea and is the result of the menstrual cycle needing time to begin again. This condition is not permanent, however, and will resolve itself with time.
