A new study suggests that the pill prevents pregnancy no matter what a woman weighs.
As long as a woman-heavy or thin - took the pill consistently, Dr. Carolyn L. Westhoff and her colleagues found, it prevented her ovaries from producing eggs.
Studies have found that obese women are 30 to 40% more likely to get pregnant
while taking the pill than were normal-weight women taking it, perhaps because excess weight might blunt its effectiveness.
In their study, Westhoff and her colleagues enrolled 226 normal-weight or obese women, randomly assigning them to take a lower or higher dose version of the pill. After three or four cycles of oral contraceptives, the researchers then had the women undergo multiple ultrasounds and blood tests to determine if their ovaries were making eggs.
97% of the 150 consistent pill users did not ovulate
, meaning they would not have gotten pregnant during that cycle. The risks of higher doses
The new findings, along with more recent clinical trials, should put to rest concerns that obese women are more likely to get pregnant while on the pill, Westhoff says. The issue has confounded doctors, she notes, because some companies have pushed physicians to prescribe higher dose formulations to their obese patients based on the idea that lower doses aren't effective.
The problem with this approach, she explains, is that obese women are at increased risk of developing life-threatening blood clots
in their veins. Taking the pill-especially at higher doses-further increases the risk these clots will occur.
Dr. Victoria Holt argues that Westhoff's research suggests obese women are less likely to achieve adequate hormone levels while taking the pill.
"It's a particular concern in the United States because obesity is increasing so rapidly," Dr. James Trussell said. "I would say that the pill is still not going to be the front line, the top tier contraception for obese women."
Methods with a lower risk of failure due to human error, like the IUD, hormonal implants, or vasectomy for the male partner, would likely be a better choice, he said.
"If a woman really doesn't want to become pregnant I would suggest combining oral contraceptives with a barrier method," Holt advised, such as condoms or a diaphragm. Are you obese? What form of contraception do you use?