There are numerous types of birth control. Most work by preventing ovulation or implantation, or a combination of the two. Oral contraceptives, Ortho Evra (the birth control patch), NuvaRing, Depo-Provera, and the Mirena IUD are all hormonal methods of contraception. ParaGard, the copper IUD, does not use hormones and works differently to prevent pregnancy.
There are two types of oral contraceptive pills, one containing estrogen and progesterone, the other contains progesterone only. Ortho Evra, a weekly patch you place on your skin, contains estrogen and progesterone and works in the same way as oral contraceptives. NuvaRing, another estrogen and progesterone based contraceptive, is similar to Ortho Evra but it is a ring that is placed inside the vagina once a month. Depo-Provera is a long-lasting synthetic hormone similar to progesterone. It is designed to last for three months but it may last even longer. It can take almost a year before Depo-Provera is completely out of your system. The Mirena IUD is a T-shaped plastic device that releases a synthetic form of progesterone. It is inserted into the uterus and can prevent pregnancy for up to five years.
During a normal menstrual cycle, your estrogen and progesterone levels tell your body when to release an egg and when to shed your uterine lining. At the start of your cycle the estrogen levels are low, which tells your body it's time to start getting eggs ready for ovulation. After you ovulate, progesterone comes into play. There are other hormones involved in this process, but estrogen and progesterone are the most important ones. Progesterone increases after ovulation but will drop off if conception does not occur. When your progesterone levels decrease, this tells your body to start your period.
By manipulating your estrogen and progesterone levels, birth control can suppress ovulation. Your body no longer controls your hormonal fluctuations. Instead, the birth control determines how much estrogen or progesterone is in your system. Your body gets used to this so when you go off of birth control it can take a while for your body to remember how to regulate itself again.
During your first cycle or two off of birth control you may notice a lot of symptoms that may feel like pregnancy symptoms. As your pituitary gland tries to take over the control of your hormones again, your body may try to ovulate and fail. The process of ovulation can cause some of the same symptoms you would feel if you were pregnant, such as breast tenderness or nausea. Normally, these symptoms are minor and can be easily missed. However, since your body is not used to all these hormonal changes, they may feel more intense.
It is also quite common for women's first period after going off birth control to be delayed. Ordinarily your menstrual period comes about 14 days after ovulation. Since ovulation is often delayed when going off of birth control, your period will also be delayed. How long depends on the type of birth control you were using. For most hormonal contraceptives, your fertility and menstrual cycles should return fairly quickly, within 2 to 3 months. Depo-Provera may take longer to get out of your system though, so it will take longer for fertility to return after taking Depo.