“The birth control pill functions primarily by preventing ovulation. The pill elevates the body’s levels of progesterone, which mimics pregnancy. The body behaves as though it is pregnant, disrupting the normal menstrual cycle and the release of additional hormones that cause a woman to ovulate. Progestin also thickens cervical mucus, which helps prevent sperm from entering the uterus. It may also prevent fertilized eggs from implanting properly in the lining of the uterus.”
https://embryo.asu.edu/pages/birth-control-pill
“The standard combined oral contraceptive consists of both hormones P and E, taken daily for three weeks, followed by a week of no pills (or placebo pills) that trigger what’s known as “breakthrough bleeding”, induced by the drop in hormones. Although the outward appearance is the same (menstrual bleeding in week 1 of a 28 day cycle) the constant high levels of P and E for a woman taking the Pill actually abolish the normal hormonal cycling that underlies ovulation. The Pill therefore replaces the normal menstrual cycle with an artificial cycle (3 weeks of “mimicking pregnancy”, followed by 1 week with breakthrough bleeding). Historically, women have been pregnant or nursing much of their adult lives and thereby suppressing ovulation naturally, suggesting that halting menstruation is not inherently harmful. Yet, many women still wonder if it is safe to eliminate menstruation using oral contraceptives.” http://sitn.hms.harvard.edu/flash/2008/issue40/