Researchers have found another downside to being obese.
Medical evidence has long suggested the birth-control pill is less effective at preventing pregnancy in obese women, and now a new study has provided fresh evidence on why the popular contraceptive might fail these women.
The study revealed that it takes about twice as long as normal for the oral contraceptive to reach an effective dose in the bloodstream of obese women.
That suggests there is a "window of opportunity" every month when the contraceptive may not be at a therapeutic level to stop a pregnancy, said one of the researchers, Ganesh Cherala of Oregon State University.
The study involved 20 women, aged 18 to 35. Half were classified as medically obese, while the others were of normal weight.
Conventional oral contraceptives gradually build up in the bloodstream each month, and reach an effective dose before the fertile stage of the cycle.
The study showed that it took an average of five days for the Pill to attain its maximum concentration in women of normal weight.
But it took an average of 10 days to reach the same level and stable concentration in the obese women. In fact, in one obese woman it took the drug more than 20 days to achieve a "steady state," according to the findings published in the journal Contraception.
So what does this mean for the chances of obese women becoming pregnant while taking an oral contraceptive?
"Those statistics are kind of murky" until more research is done, Dr. Cherala said.
Many women become fertile roughly half-way through their monthly cycle. But some have very irregular cycles, which may increase the odds of pregnancy if the oral contraceptive is below a therapeutic dose at the critical time.
The study's lead author, Alison Edelman of Oregon Health and Science University, noted that the Pill is 92 per cent effective at preventing pregnancy in women of normal weight.
"We don't have enough information yet to recommend changes to clinical practice," Dr. Edelman said. But if a woman wants to be extra safe, she should consider a contraceptive option that works regardless of weight, such as an intra-uterine device. "It's long-acting, very cost-effective and reversible."