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How could she get pregnant while on birth control pills?

By Courtney Humphries
May 17, 2010

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Q. I got pregnant while on birth control pills — how could that happen?

A. The birth control pill turns 50 this month, and although it’s famous enough to simply be called “the pill,’’ there are still misunderstandings about it. Many women have heard that the pill is 99 percent effective, but they assume that the reason it doesn’t achieve a perfect score is because women skip pills or don’t take them at the right time. In fact, says Robert Barbieri, chief of obstetrics and gynecology at Brigham and Women’s Hospital, birth control pills “are FDA tested in general to achieve a 99 percent success rate with very good use. So we’re using a medication that tends to have failure of at least one in 100 even if it’s used perfectly.’’ In such cases, the reason for the failure is that the hormone levels are kept low enough to reduce side effects like blood clots, while slightly sacrificing effectiveness.

For a given individual, it’s difficult to know why her birth control might have failed. Skipping pills is certainly a common reason. But some people also metabolize the pill more quickly than others, which results in the medication being cleared more quickly from the body. Certain medications, particularly the tuberculosis-treatment drug rifampin, can potentially interfere with pill metabolism.

Body weight could also have an effect. Barbieri says that birth control pills are not tested on women who are obese, and some observational studies of women taking the pill have detected a slightly higher failure rate in those women.

Women who have experienced pill failure might benefit from newer types of pills that require spending fewer days off hormones than the typical seven days. Barbieri says that women who miss pills often do so at the beginning of a cycle, which can be a problem when they have already spent a full week without hormones. If you’re not willing to live with that 1 percent chance of pregnancy, he recommends using condoms in addition to the pill, or choosing an IUD, which has a failure rate as low as 0.1 percent.

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