Women have long blamed the birth-control pill for packing on extra pounds. Now it turns out excess weight may be to blame when the pill fails to prevent pregnancy. A study of Seattle...

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Women have long blamed the birth-control pill for packing on extra pounds. Now it turns out excess weight may be to blame when the pill fails to prevent pregnancy.

A study of Seattle women found that those who are overweight or obese have a 60 to 70 percent higher risk of getting pregnant while on the pill than thinner women do.

That could mean an additional two to four unplanned pregnancies per 100 women a year due to being heavy.

“It’s another hazard of obesity,” lead researcher Victoria Holt said.

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The finding, published today in the journal Obstetrics & Gynecology, may prompt overweight women to rethink their reliance on the pill. “Some women may want to add a backup measure, like a condom,” said Holt, an epidemiologist at the Fred Hutchinson Cancer Research Center.

She analyzed data on nearly 250 Group Health Cooperative patients who became pregnant while on the pill and compared them to more than 500 pill users who didn’t get pregnant.

The chance of pregnancy jumped by 60 percent among women with a body-mass index (BMI) of 27.3 or higher. That translates to a woman who’s about 5 feet, 4 inches and 160 pounds. BMI is a rough measure of body fat based on height and weight, and the government considers people with a BMI over 25 overweight and over 30 obese.

Pill users with a BMI over 32.2 had a 70 percent higher risk of pregnancy than women at a healthy weight.

When researchers focused only on women who had perfect pill use, meaning they didn’t miss any doses, the link between weight and contraception failure was even stronger. Heavy women had more than double the risk of pregnancy.

The link is especially concerning, because pregnancies often are more complicated in overweight women. “They’re more likely to have high blood pressure, gestational diabetes, preeclampsia, difficulty in labor and delivery and are more likely to have a C-section delivery,” Holt said.

The study didn’t address the biology behind the link, but the researchers propose possible theories. First, the more a person weighs, the higher the metabolic rate, which may shorten the pill’s activity. Second, heavier people have more liver enzymes, which means the pill may be cleared faster from the system.

Another possibility is the more fat tissue a woman has, the more likely fat-soluble hormones in the drug will get stashed away instead of circulating in the bloodstream.

“It may be that all of these things are contributing to the pill not working as well in overweight women,” said Delia Scholes, a researcher at Group Health’s Center for Health Studies and co-author of the report.

The birth-control pill is the leading form of contraception in the U.S., used by nearly 12 million women. Two other forms of hormonal contraception, the Norplant implant and the patch, already have been shown to be less effective in heavy women.

Today’s oral contraceptives contain much lower doses of hormones than those on the market a few decades ago. This reduces side effects and health risks but may mean the pill isn’t potent enough for larger people.

Some physicians had suspected as much, said Dr. Ruth Krauss, an OB-GYN at Group Health who wasn’t involved in the study. “When we give many medications — pain medicine or antibiotics, for instance — we give more to larger people because it’s spread through a larger blood-circulation system.”

But giving heavier women a much higher dose of the pill wouldn’t be advisable, she said, because they’re already more at risk for potential side effects, such as blood clots, heart disease and stroke.

Instead, doctors should mention this increased pregnancy rate to their heavier patients. “It tips the equation a bit,” Krauss said. “This study might prompt more frequent use of nonhormonal contraception — like IUDs.” And using a condom and spermicide might be almost as effective as the pill for some women, she said.

Julia Sommerfeld: 206-464-2708 or jsommerfeld@seattletimes.com