Some women are getting pregnant to avoid getting fat. According to a review published in a recent issue of the journal Contraception, concern about weight gain is among the most common reasons people quit the pill (or decide not to take it in the first place), and most women who discontinue the pill switch to something less effective or nothing at all. Some of those women end up with unintended pregnancies simply because they feared a few extra pounds.
The notion that birth control pills make you fat is just part of what the paper's lead author, physician David Grimes of the University of North Carolina, calls an elaborate mythology, one "fueled by rumor, gossip and poor-quality research." As it turns out, there's never been convincing evidence that the pill in any of its forms provokes weight gain. Another recent review paper found little evidence of weight gain among users of progestin-only pills, and a similar study from 2008 likewise failed to find any connection between weight gain and pills that contain progestin and estrogen.
Some studies hint that the pill might even have the opposite effect. According to a 1997 study, users of birth control pills have a basal metabolic rate almost 5 percent higher than people who have never used the pill, and a small 2009 trial concluded that the pill reduced body fat in women who took it. A study published last year tracked nearly 500 women aged 15 to 19 over several years and found that while all the teens gained some weight, those who used birth control pills gained about a pound less than those who didn't. (Young women who used injectable hormonal contraceptives like Depo-Provera did pack on extra weight—they gained twice as much, on average, as everyone else.)
If the pill doesn't cause weight gain, how did it get its reputation? Chalk up some of it to folklore about hormones, which are regularly blamed—sometimes with just cause—for everything from moodiness to bloating and breast tenderness. It's not just sexist thugs who joke that female hormones make women bitchy and unappealing around their cycles; plenty of women make PMS jokes, too. The pill is often said to mimic pregnancy, and everyone knows that pregnant women experience huge changes to their bodies, including additional pounds.
Perhaps the most powerful factor in the spread of misinformation is the pill packets themselves. The labels of most oral contraceptives list "weight gain" among their common side effects. If the drug companies themselves are telling us that the pill makes us fat, why shouldn't we believe them? The warnings on the label are not always so trustworthy; in many cases, they're simply a function of the quirky way in which side effects are reported in clinical trials. Participants are asked to report any symptoms they notice during the study, and those that turn up most often may end up being listed even if there's no proof they were caused by the medication.
As a result, almost any trial will turn up an array of what doctors call "nonspecific" symptoms—vague complaints that do not signal a specific condition. "They're the aches and pains of daily life," Grimes says, things like headaches and moodiness and, yes, weight gain. Weight gain is especially pervasive—the average adult gains about a pound per year—so even if people taking a medication do put on a few pounds, it's hard to pin the effect on a particular drug. Medications proven to cause weight gain, like antidepressants, antipsychotics, allergy medications, and steroids generally lead to much greater increases in body weight.