Combination birth control pills, also known as the pill, are oral contraceptives that contain estrogen and a progestin.Combination birth control pills suppress ovulation - keeping your ovaries from releasing an egg. Combination birth control pills also cause changes in the cervical mucus and the lining of the uterus (endometrium) to keep sperm from joining the egg.Different types of combination birth control pills contain different doses of estrogen and progestin. Some combination birth control pills, called continuous or extended-cycle pills, allow you to reduce the number of periods you have each year. If you'd like to use combination birth control pills, your health care provider can help you decide which type is right for you.