How the Pill Works
Female Menstrual Cycle | Menstrual Cycle with Pill
Alterations to the cycle by the birth control pill:
Days 1 - 5
The pill uses the body's negative feedback system to prevent ovulation and implantation of an embryo, in the same way the hormones provided by the placenta halt the female cycle during pregnancy. Women using birth control pills must remember to take one each day.
Synthetic estrogen and progestin (a form of progesterone) in the pills travel through the bloodstream to the hypothalamus and the pituitary gland. The estrogen suppresses production of FSH in the pituitary gland so follicle maturation doesn't occur.
Days 6 - 10
The pituitary gland steps down production of FSH. Without FSH from the pituitary gland, no dominant follicle develops and there is no increase in estrogen. The uterine lining doesn't thicken.
Days 11 - 13
Constant low levels of progestin send a signal to the hypothalemus and pituitary gland to prevent the LH surge.
Without the LH surge, ovulation doesn't occur and no ovum is released. The constant level of progestin makes the lining of the uterus inhospitable to implantation of an embryo. The cervical mucus remains thick.
Days 15 -21
The continued delivery of estrogen and progestin maintains the suppression of FSH and LH.
Days 22 - 28
Placebo pills without estrogen and progestin allow for the breakdown and release of the thin uterine lining as a withdrawal bleed, similar to menstruation.