rerun image Although women are grateful for this relatively simple and reliable form of contraception, most of them readily admit that they don't know much about how oral contraceptives actually work.

When you take oral contraceptives, the hormone thins out the lining of the uterus making it unable to host egg implantation. The mucus in the cervix thickens, making it tough for sperm to swim through.

Most birth control pills contain two hormones, estrogen and progestin. Pills that contain both estrogen and progesterone are called combined contraceptives. There is also another type of pill that contains progestin-only, called the mini-pill.

Progestin-Only Pill

Progestin-only birth control pills are a popular choice for many nursing and non-nursing women immediately postpartum. Also called the mini-pill, this method is less effective than combination pills. But, it is often recommended after delivering your baby because combination pills increase your risk of developing blood clots.

They may also reduce the risk of triggering migraine headaches compared to pills with estrogen.

Combination Oral Contraceptive Pills

The same kind of estrogen is in all low dose combination pills. The exact dose of estrogen may have an impact on the kind of side effects you experience.

If you have a history of blood clots, or have high blood pressure, your doctor may advise that you take the progestin-only pill. Also, many organizations are concerned that combined oral contraception may suppress milk production, so they do not recommend them in breastfeeding women. But, currently there is not enough evidence to show that birth control pills affect how much milk your body makes.

Monophasic vs. Triphasic

Within the family of estrogen-containing pills, there are two main types: monophasic and triphasic. As the names suggest, monophasic pills provide the same level of hormones throughout the pill cycle. Triphasic pills also induce a steady state of hormones, but at three different levels during the cycle. The two types are equally effective for pregnancy prevention.