What is the Progesterone-Only Pill?

The progesterone-only pill, also called the mini pill or the progestin-only pill, differs from other types of birth control pills in that it contains only one hormone, a synthetic form of progesterone. Other birth control pills typically contain both estrogen and progesterone. It also contains a smaller amount of progestin than other methods. The progesterone-only pill works in a way similar to other birth control pills but has fewer side effects and is safe for nursing mothers to use.

Birth control pills, including the progesterone-only pill, prevent pregnancy by altering a woman's body in three ways. The primary way a progesterone-only pill works to prevent pregnancy is by thickening a woman's cervical mucus, which makes it very difficult for sperm to find its way to an egg. In some cases, the mini pill can also prevent ovulation so that a woman does not release an egg each month. In case an egg does become fertilized, the pill makes it more difficult for the egg to implant on the wall of the uterus.

Since they contain no estrogen, progesterone-only pills have less side effects than other hormonal contraceptives. Some women experience nausea and vomiting as well as headaches from pills that contain estrogen. Pills with estrogen also increase a woman's risk for blood clots and stroke. Women who smoke or are more than age 35 may find taking the progesterone-only pill safer.

Birth control pills with estrogen can also lower the amount of milk a breastfeeding mother produces and affect the quality of the milk. Typically, the progesterone-only pill does not interfere with milk production. A small amount of the hormone may be found in the mother's milk, but not enough to cause harm to a baby.

The chance of pregnancy when a woman takes the progesterone-only pill is about two in 100. The pill needs to be taken at the same time every day to work properly. The amount of progestin in the pill stays in the body for only about 24 hours, so a woman risks pregnancy if she misses a dose or takes the pill too late. After a woman stops taking the mini pill, her fertility should return right away.

After a woman starts the mini pill, she should still use another method of birth control for at least a month until her body adapts to the hormone. If she takes the pill more than three hours after the scheduled time, she should use an alternate birth control method, such as a condom, for a least two days. Missing a dose means she has to start over again and use a back-up method for a least a month.