Studies were also difficult to interpret and compare because they involved many varieties of pills and regimens, which complicated consensus.
The paper, published in 2008, concluded: "Given the number of contraceptive drugs, doses and regimens, the possibility that one or more combination contraceptives could cause weight gain cannot be eliminated.... The evidence was not strong enough to be sure that these methods did not cause some weight gain."
More recently, in a study published last month in Human Reproduction, researchers assessed whether oral contraceptives caused weight gain in rhesus monkeys.
The yearlong study, conducted at the Oregon Health and Science University's National Primate Research Center, focused on rhesus monkeys because their reproductive systems are very similar to those of humans. While conceding that the subjects were monkeys, not people, the authors said that the findings "suggest that worries about weight gain with pill use appear to be based more on fiction than fact."
A problem with averages
Another reason women and doctors view the potential for weight gain differently is because studies average data. Many women don't gain weight on oral contraceptives, some do and some lose. When all that averages out, the net result is little to no overall gain. But tell that to the women who suddenly can't zip their jeans.
For about 10% of women, those with polycystic ovary disease, birth control pills are a one-way ticket to weight loss — treating the underlying hormonal imbalance.
"These women tend to be overweight, and in their case, when they go on birth control pills, many lose weight," says Azziz, former director of Cedars-Sinai Medical Center's androgen-related disorders center.
But women who don't have the condition can gain weight. In Azziz's clinical experience, about 80% of women do not see a change in weight, 15% gain weight (generally 3 to 5 pounds) and 5%, mainly the polycystic ovary patients, lose weight.
"The way we really need to interpret the research is by realizing there is no average woman," says Berenson, also a professor of obstetrics and gynecology.
She believes that, in general, if a woman does gain weight on oral contraceptives, the pills are probably not to blame.
"Whether on oral contraceptives or not, women during their child-bearing years gain on average 10 pounds a decade," Berenson says. She advises patients who gain weight to examine their diet, exercise and behavior.
Nonetheless, if birth control pills do trigger weight gain in some women, science has an explanation.
Hormones a cause
Progesterone can increase appetite, says Azziz, and estrogen leads the body to retain fluid, which can account for 2 to 3 pounds.
Further, estrogen can make thyroid hormone and testosterone less available to the body's tissues, says Stanley Korenman, an endocrinologist and professor of endocrinology at the UCLA David Geffen School of Medicine, potentially, if only slightly, affecting body composition.
When estrogen binds with thyroid, the body recognizes that it needs more thyroid and compensates by telling the thyroid gland to make more.
The suppression of testosterone is why birth control pills improve the skin of girls and women who have hormonal acne. The fall in testosterone may lower sex drive but shouldn't affect weight, he says.
Estrogen may, however, be responsible for the change in body composition some women notice, Korenman adds. Studies show that, although weight may not change when a woman takes oral contraceptives, body composition can. Women on oral contraceptives tend to increase fat and decrease lean muscle mass. Because muscle weighs more than fat, women can get bigger and flabbier but not heavier, though, he adds, the shift will be small.
So what's a woman to do? Know and understand the risks, the reasons behind them and whether they are worth the benefits. If you experience a slight weight gain, which is likely due to water retention, Azziz recommends asking your doctor for a mild diuretic.
And all experts recommend that if you choose to be on the pill, go on the lowest effective dose.
For Webb, who now relies on barrier methods of contraception, that dose is zero.