Hormonal birth control suppress other hormones that are needed for female fertility. Both oral contraceptives and other forms of hormonal birth control use either a combination of synthetic estrogen and progesterone or progesterone alone. The estrogen, according to the University of Michigan Health Center, suppresses the body's production of a hormone called follicle stimulating hormone, which promotes the maturation of a mature follicle in the ovaries. Follicle maturation is necessary for the release of an egg. The other hormone, progesterone, blocks production of luteinizing hormone, which is needed for egg release (ovulation). Consequently, when a person goes off birth control, the levels of these two hormones (follicle stimulating hormone and luteinizing hormone) go up.
Hormonal birth control also affects the body's tissues. The cervix is an organ at the back of the vagina that makes a mucus and is the gateway to the uterus. Progesterone makes cervical mucus thicker, so women going off of birth control may notice that these secretions are thinner than they were previously. It is easier for sperm to get through this thinner cervical mucus, making it easier to get pregnant.
Progesterone also makes the lining of the uterus somewhat thinner. Hormonal birth control cessation causes the uterine lining to become thicker (which is also enhanced by increased levels of follicle stimulating and luteinizing hormone). As the Mayo Clinic notes, this increase in thickness of the lining of the uterus can cause some mild bleeding (also known as spotting) after going off of birth control.
Return of Fertility
The main effect of going off of birth control is, of course, the return of the menstrual cycle (and the resumption of fertility). Although women taking the pill make experience some bleeding, but this is termed breakthrough bleeding and is not a true period. While a woman is taking hormonal birth control, her menstrual cycle is disrupted and she is infertile. Once the synthetic estrogen and progesterone is out of the body, the endocrine system (which produces the hormones that cause the menstrual cycle) is able to begin making the hormones necessary for ovulation and fertility. In some cases, women do not have their menstrual cycle return for several months (a condition known as post-pill amenorrhea). This is because it can take some time for the body to resume hormone production. This condition is temporary, however, and over time the patient's fertility will return to normal.