Ask Dr. K: Sexually active women must find right birth control pill

Birth control pills can be divided into two basic categories: combined pills and minipills. Most birth control pills are combined pills. They contain a combination of two types of female sex hormones: the hormone estrogen and a progestogen (a group of hormones that includes progesterone). Minipills contain a progestogen, but no estrogen.

The estrogen in combined birth control pills prevents ovulation, the release of a mature egg from a woman's ovary. Combined birth control pills also thicken cervical secretions, creating a barrier that makes it difficult for sperm to get into the uterus. They produce changes in the tubes that carry the egg from the ovaries to the uterus. And they make it hard for a fertilized egg to implant in the uterus. These effects are caused by the progestogen.

Combination birth control pills are 98 to 99 percent effective in preventing pregnancy. The more estrogen, the better the birth control effect. Unfortunately, the more estrogen, the higher the likelihood of side effects such as high blood pressure or headaches.

Minipills do not prevent ovulation as effectively as combination pills because they don't contain estrogen. They mainly act by thickening secretions of the cervix and making it hard for a fertilized egg to implant in the uterus. They are about 98 percent effective. Minipills need to be taken exactly as directed. Even missing one pill can greatly reduce their effectiveness.

Combination birth control pills are available in three subtypes: monophasic, biphasic and triphasic. Monophasic pills contain a constant amount of estrogen and progestogen. With biphasic and triphasic pills, the dose varies over the course of the month.

For most women, monophasic birth control pills are a good first option. They work as well as the more expensive and more complicated biphasic and triphasic products.

Minipills are a good option if you have a reason to avoid estrogen. The most common reasons are a tendency to develop blood clots, smoking and current breastfeeding. That's because estrogen increases the tendency for blood clots. In smokers, even if they've never had a problem with blood clots in the past, starting on estrogen increases the risk for clots -- and of the heart attacks and strokes caused by them.

Estrogen reduces the amount of breast milk, so estrogen-containing birth control pills normally are not recommended in women who are breast feeding.

It may take some trial and error to find the best birth control pill for you. Try to achieve the right hormonal balance -- just enough to prevent pregnancy, but not enough to cause side effects.

Dr. Komaroff is a physician and professor at Harvard Medical School. To send questions, go to AskDoctorK.com, or write: Ask Doctor K, 10 Shattuck St., Second Floor, Boston, MA 02115.