Women who are overweight or obese have a much higher chance of becoming pregnant because their Pill has failed, researchers have found.
Overweight women are advised to use barrier contraceptives
Overweight women were 60% more likely to fall pregnant while on the Pill.
Obese women were 70% more likely, found a study in Obstetrics and Gynaecology by a team from the Fred Hutchinson Cancer Research Centre in Seattle.
It suggested that of 100 women on the Pill, an extra two to four would fall pregnant due to being overweight.
The Pill is usually estimated to be over 99% effective. This means that less than one woman in 100 will get pregnant in a year.
However, that figure relates to perfect Pill use. Actual failure rates are estimated to run at around 6%.
Researchers compared the weight and body-mass index (BMI) of 248 women who became pregnant while on the Pill, and compared them to a group of 533 women of the same age who were taking oral contraceptives but who were not pregnant.
BMI is calculated by dividing your weight in kilograms by the square of your height in metres.
A BMI of over 25 is considered overweight and one of 30 or above is considered obese.
It was found that the link between carrying extra weight and contraceptive pill failure became evident in women whose BMI was 27.3 or higher - equivalent to a 5ft, 4in woman who weighs 160lbs (11st 6lb, or 72.5kg) or more.
The researchers say their study did not look at why this link should exist.
But they suggest a higher metabolism, linked to extra weight, could be a factor, because it can shorten the duration of a medication's effectiveness, or that hormone levels in the Pill may not be high enough for larger women.
In addition, they said the more overweight or obese a woman is, the more liver enzymes they have to clear medications from the body, causing a drop in the amount of a drug circulating in the blood.
The researchers say another explanation could be linked to the fact that the active ingredients in oral contraceptives, oestrogen and progesterone, are stored in body fat - so the more likely the drug is to become trapped in the fat instead of circulating in the bloodstream.
Dr Victoria Holt, who led the study, said: "These results 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."
She added: "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."
Dr Holt said women who are overweight should not ask for a higher dose Pill, because they were already at a higher risk of cardiovascular disease which contraceptive hormones increase even more.
She suggested women who had completed their families should consider a permanent form of birth control such as sterilisation, and those who still wanted to have a family considered using a back-up form of birth control, such as a condom, as well as the Pill.
She added: "I think losing weight, if one is substantially overweight, is a terrific idea for many health reasons and a laudable goal."
But she said "I also acknowledge that it is often difficult to do."
Geoffrey Chamberlain, professor of obstetrics and gynaecology at the University of Wales College of Medicine, said: "The Pill is not so effective in overweight women. The hormones get absorbed into the fat, so the blood concentration and the effect on the ovaries is lower.
"Therefore, it may be advisable for women who are overweight to use other methods of contraception such as barrier methods or an intrauterine device."