WEDNESDAY, Dec. 29, 2004 (HealthDayNews) -- Women who are overweight or obese are more likely to get pregnant while taking birth control pills than women of normal weight are, new research finds.
In the group of women studied, 5 percent of overweight women taking the Pill got pregnant each year vs. 3 percent of normal weight women, said study author Victoria Holt.
The study, which the researchers believe is the first of its kind, was funded by the National Institute of Child Health and Human Development. It appears in the January issue of Obstetrics and Gynecology.
The finding is potentially significant because overweight women have a higher chance of complications while pregnant, including gestational diabetes and high blood pressure.
But while obese or overweight women in the study were 60 percent to 70 percent more likely to get pregnant while on the Pill, other experts emphasized that the actual chances of conceiving were not alarming.
"The absolute risk is low," said Dr. Ralph Dauterive, head of obstetrics and gynecology at the Ochsner Clinic Foundation in New Orleans. "Who knows why this is occurring. The solution is to add condoms, an IUD or no sex."
Oral contraceptive trials have reported pregnancy rates of 0.5 percent or lower, but national surveys have put the actual number much higher. There are more than 400,000 pregnancies among users of oral contraceptives in the United States each year, the study said.
In the 40 years since the first birth control pills hit the market, the amount of the hormone estrogen they contain has decreased fivefold.
"Maybe we're at a point that it's not enough for some women," said Holt, who is a member of the Public Health Sciences Division at Fred Hutchinson Cancer Research Center in Seattle.
The genesis of the new study reaches back several years to a meeting of a U.S. Food and Drug Administration panel convened to discuss the possibility of removing higher-dose birth control pills from the market. At the meeting, one of the new study's co-authors overheard another panel member, who was also a physician, say, "Don't do that. All of my overweight patients are going to get pregnant.'"
Holt is a former pediatric nurse who knows from experience that the dosage of virtually every drug administered is determined by patient weight. "It made perfect sense to me," Holt said. "We also knew that obesity is getting to be more common, so there is a bigger variation in adult women's weight, from 100 pounds to 300 pounds."
Holt and her colleagues compared the weight and body mass index (BMI) of 248 women who became pregnant while using birth control pills between 1998 and 2001, to 533 control subjects who used oral contraceptives but did not become pregnant. All the participants were enrolled with a health maintenance organization in Seattle.
The risk of pregnancy was nearly 60 percent higher in women with a BMI greater than 27.3, and more than 70 percent higher in those with a BMI in excess of 32.2.
A BMI of 27.3 or more is roughly equivalent to a 5-foot, 4-inch woman weighing 160 pounds or more. A person with a BMI greater than 25 is considered overweight; a BMI above 30 is obese.
Among consistent birth control pill users, the risk of pregnancy was more than 70 percent higher in women weighing more than 165 pounds and nearly double in women weighing more than 190 pounds.
The researchers said there are several possible explanations -- though none proven -- for the findings.
First, overweight and obese people have a higher metabolism, which means it's possible the Pill is "used up" more quickly in these women, Holt said.
Also, overweight women have more liver enzymes, which help metabolize the hormones in birth control pills. "That would also cause the pills to be used up more quickly," Holt said.
The final possible explanation has to do with body composition. "Women with high BMI are more likely to have more fat and the hormones in birth control pills are fat-soluble," Holt explained. "The hormones may get trapped in the fat rather than entering the bloodstream."
Since the study authors found a stronger connection between BMI and pregnancy than weight and pregnancy, they tend to gravitate toward the last two explanations.
However, Holt added, the answer isn't for heavy women to take high-dose pills. "Heavier women have cardiovascular risk factors and a higher dose might increase that risk further," she said.
Women who have completed their families might consider a permanent birth control method, such as tubal sterilization. And, as Dauterive noted, women still considering more children should think about adding a back-up method such as a condom or IUD.
And all women should make sure they take birth control pills as directed and without skipping any doses. "We know that increases the risk of pregnancy," Holt said.
More information
Calculate your body mass index at the National Heart, Lung, and Blood Institute.
