Being overweight dramatically increases the chances of women on the Pill getting pregnant, scientists have revealed.
A woman who is overweight or obese is 60% to 70% more likely to conceive while taking an oral contraceptive than one who is slim, researchers found.
The reason for the higher pregnancy risk is not fully understood, but may be linked to the way contraceptive drugs are processed in people with excess body fat.
Scientists in America compared 248 women who became pregnant on the Pill with an age-matched comparison group of 533 non-pregnant women using the same form of birth control.
Research leader Dr Victoria Holt, from the Fred Hutchinson Cancer Research Centre in Seattle, said: "The results of our study represent yet another reason why obesity is a health hazard.
"Overweight and obese women have a significantly higher risk of getting pregnant while on the pill than women of normal weight, and this translates into a substantial number of unplanned pregnancies.
Obesity-related complications
"This higher risk of pregnancy also translates into a higher number of obesity-related complications of pregnancy, which range from gestational diabetes and high blood pressure to Caesarean delivery."
The scientists measured the women's body mass index (BMI). This relates height and weight and is obtained by dividing weight in kilograms by height in metres squared.
BMIs of 18.5 to 24.9 are considered normal, 25 to 29.9 overweight, and anything above 30 obese. The risk of getting pregnant while on the Pill jumped dramatically for women in the top quarter of BMI measurements.
The effect first surfaced in women whose BMI was 27.3 or higher - equivalent to a 5ft 4in woman weighing 160 pounds or more.
These women faced a 60% greater risk of pregnancy. For those considered obese, with a BMIs of 32.2 or greater, the increase in risk rose to 70%.
Several biological mechanisms could account for the pattern, the researchers report in the January issue of the journal Obstetrics and Gynaecology.
Increased metabolism
One possible explanation is increased metabolism. "The more a person weighs, the higher their basal metabolic rate, which can shorten the duration of the medication's effectiveness," said Dr Holt.
Heavy individuals also had more liver enzymes to clear chemicals from the body, which could cause a drop in circulating blood levels of the drug.
A third theory was based on the fact that the hormones in contraceptives are stored in body fat. Fat could "trap" the contraceptive and prevent it from working.
Today's oral contraceptives also contained relatively low hormone levels compared with those first introduced four decades ago.
"Hormone levels have decreased five-fold in an effort to reduce unwanted side effects ranging from weight gain to risk of blood clots and stroke," said Dr Holt. "Today's pill dose is high enough for most women but may not be adequate for all."
However she warned against overweight women demanding a higher-dose Pill, pointing out that they were already likely to be at greater risk of heart disease.
A high-dose oral contraceptive would increase their heart disease risk still further.
