The pill works by borrowing the body's own hormonal off switch
Every menstrual cycle contains a built-in brake: rising estrogen and progesterone tell the pituitary gland to stop releasing the hormones that ripen an egg. The combined pill, approved in the United States in 1960, simply keeps that brake pressed, so ovulation never gets its signal.
A natural cycle runs about 28 days. In the first half, the pituitary releases follicle-stimulating hormone, which makes a follicle grow in the ovary; the follicle in turn produces estrogen, thickening the uterine lining. Around mid-cycle a brief surge of luteinizing hormone releases the egg. The empty follicle then becomes the corpus luteum, pouring out estrogen and progesterone that hold the lining in place and damp down the pituitary. When it breaks down, hormone levels fall and menstruation begins.
The combined pill contains a synthetic progestin plus an estrogen. The progestin slows the pulses of gonadotropin-releasing hormone from the hypothalamus, cutting follicle-stimulating hormone and sharply reducing luteinizing hormone, so no egg matures or is released. Estrogen was first added mainly to steady the uterine lining and reduce breakthrough bleeding, but it turned out to suppress follicle development too. Progestins also make cervical mucus thicker and less watery, obstructing sperm. A typical pack spans 28 days, with 21 days of active hormone tablets.
Pharmacologists sort combined pills into generations by their progestin. Early formulations from the 1960s used high hormone doses; later generations reduced estrogen and introduced new progestins such as levonorgestrel, desogestrel and, in the fourth generation, drospirenone. The literature defines these groupings inconsistently.
More than 100 million women worldwide use combined pills, about 9 million of them in the United States, where 12.6 percent of women aged 15 to 49 reported using them between 2015 and 2017, second only to female sterilization. The pill appears on the World Health Organization's List of Essential Medicines and is widely described as a catalyst of the sexual revolution.
Source: Combined oral contraceptive pill
