The pill keeps women from getting pregnant because the menstrual cycle is controlled by hormonal changes in the body. Let's take a look at the menstrual cycle.
Although its actual length can vary from woman to woman, the menstrual cycle is generally 28 days long and follows a few basic phases that are all triggered by the release of different hormones.
First, the pituitary gland sends out follicle-stimulating hormone (FSH). Just like the name implies, FSH stimulates follicles in the ovaries to grow. The follicles release the hormone estrogen, which sets off a chain reaction. Estrogen triggers the pituitary gland to secrete gonadotropin-releasing hormone (GnRH), which in turn triggers a rise in the secretion of luteinizing hormone (LH). Generally, one of the ovarian follicles dominates the others in size and growth. Estrogen and LH continue to rise, which prompts the uterus to build up its endometrium, the thickened uterine lining, and causes changes in the vaginal mucus that make it a better environment for the sperm.
A rise in LH causes the dominant follicle to mature into an ovum, or egg, while the immature follicles dissolve. The egg releases from the ovary (a process called ovulation) and enters a fallopian tube. If it goes unfertilized, the egg eventually dissolves. If sperm are present, the egg may be fertilized in the fallopian tube. Then it travels down into the uterus and implants in the endometrium. After the egg is released, a structure in the ovary known as the corpus luteum produces hormones such as progesterone and estrogen. These hormones help make the endometrium suited for the egg's implantation and cause changes in the uterus to support the egg's growth.
Next, we'll look at what the hormones in the pill do.