Birth control pill equally effective for women regardless of their weight

August 16, 2010

The first study to compare the effectiveness of the birth control pill in women with marked weight differences has found that the pill works equally well in women with obesity and thinner women. This new finding by physician-scientists at NewYork-Presbyterian Hospital/Columbia University Medical Center refutes a long-held conviction among many doctors that the pill may not reliably prevent pregnancy in women who are overweight or obese.

With obesity a significant health issue in the United States -- the U.S. government estimates that nearly 65 percent of adult women ages 20 and older are overweight or obese -- the reliability of the in this population is critical, especially since pregnancy itself is riskier among women with obesity.

"As a physician, I am relieved by the results of this study. When I prescribe to my patients with obesity, I can feel confident that I am giving them something that will work," says principal investigator Dr. Carolyn Westhoff, professor of obstetrics and gynecology and director of the Division of Family Planning at Columbia University College of Physicians and Surgeons, and an obstetrician/gynecologist at NewYork-Presbyterian Hospital/Columbia University Medical Center.

In the study, published in the August issue of the journal Obstetrics & Gynecology, Dr. Westhoff and her colleagues did not rely, as previous studies had, on women's recollections of how much they may have weighed at a time when the pill had failed and they became pregnant.

"We wanted to study what was actually happening in the ovaries of women and not depend on memory, which is notoriously faulty," Dr. Westhoff says.

Dr. Westhoff and her colleagues designed a prospective study where 226 women of normal weight or who were overweight, and between the ages of 18 and 35, were randomly assigned to take either a lower- or higher-dose version of the pill. The researchers purposely used the different dose levels to assess whether heavier women required higher dosing, as has been previously believed.

After three or four months of using the oral contraceptives -- the time it usually takes for a woman's body to acclimate to the pill -- the women had multiple ultrasounds and blood tests to determine if ovulation was being suppressed. The goal of oral contraception is to suppress ovulation.

Of the 150 women who used the pill consistently, three of the 96 women with normal weight ovulated, as did one of the 54 women with obesity. The researchers also found that when women were not taking the pill regularly, they ovulated with greater frequency.

"Our findings strengthen the message to patients that the pill will only work if it is taken every day. Weight does not seem to have an impact on suppression of ovulation, but consistency of pill-taking does," Dr. Westhoff says.

Importantly, the lower-dose pill seemed to be as effective as the higher-dose pill in suppressing ovulation in women with obesity. This is a crucial finding because women with obesity are at greater risk for developing blood clots from taking either type of pill, although the overall risk is small.

"Knowing that the lower dose works as well as the higher dose will allow physicians to not only help with obesity avoid unwanted pregnancies, but also protect them from the possible health risks associated with higher doses," Dr. Westhoff says.

"For a woman to fear relying on her oral contraceptive to prevent an unwanted pregnancy is a huge burden. This study should put those fears to rest," Dr. Westhoff says.

Provided by: New York- Presbyterian Hospital/Columbia University Medical Center

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