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Birth control pills are medication used to prevent pregnancy. They contain hormones that suppress ovulation. The most effective type is the combination pill which contains both estrogen and progestin – a synthetic form of progesterone. These 2 hormones interfere with the normal menstrual cycle to prevent ovulation.
The menstrual cycle refers to the monthly events that occur within a woman’s body in preparation for the possibility of pregnancy. Each month, an egg is released from an ovary in a process called ovulation. At the same time, the lining of the uterus thickens, ready for pregnancy. If fertilization does not take place, the lining of the uterus is shed in menstrual bleeding and the cycle starts over. The menstrual cycle is under control of multiple hormones secreted by the hypothalamus, pituitary gland, and ovaries. Basically, the hypothalamus produces gonadotropin-releasing hormone, GnRH; the pituitary secretes follicle-stimulating hormone, FSH, and luteinizing hormone, LH; while the ovaries produce estrogen and progesterone. These hormones are involved in a REGULATORY network that results in monthly cyclic changes responsible for ovulation and preparation for pregnancy.
The 2 hormones that are required for ovulation are: FSH, which starts the cycle by stimulating immature follicles to grow and produce a mature egg; and LH, which is responsible for the release of the egg from the ovary – the ovulation event itself. Two other hormones, estrogen and progesterone, are at high levels after ovulation, in the second half of the cycle. They suppress FSH and LH during this time, preventing the ovaries from releasing more eggs. If fertilization occurs, estrogen and progesterone levels REMAIN HIGH throughout pregnancy, providing a continuous suppression of ovulation. On the other hand, in the absence of pregnancy, their levels FALL, causing menstrual bleeding.
The levels of estrogen and progesterone in the combination pills mimic the hormonal state after ovulation, tricking the ovaries into thinking that ovulation has already occurred; FSH and LH are constantly suppressed, no egg is matured or released.
The pills are taken every day for three weeks, followed by one week of placebo pills containing no hormones. During the week of placebos, estrogen and progesterone levels fall, triggering a so-called withdrawal bleeding, or fake periods. The bleeding serves as a convenient indication that fertilization did not happen, but it is not required for birth control. In fact, there exist continuous-use contraceptive pills with less or no placebos, resulting in less or no menstrual periods. These pills are particularly beneficial for women who suffer from menstrual disorders such as excessive menstrual bleeding, painful menstruation and endometriosis.
For lactating women, or those who cannot tolerate estrogen, there are mini-pills that contain only progestin. These are not as effective as combination pills at preventing ovulation. Their effect relies more on the ability of progestin to promote secretion of a thick cervical mucus to obstruct sperm entry.