There’s a lot of talk about being overweight or obese in America and how it can affect people’s health in all kinds of ways… but does it affect women’s birth control choices? The answer, as usual, is complicated.
BMI… What’s in a number?
Being overweight or obese is usually defined by body mass index (BMI), which is an estimation of a person’s proportion of body fat based on weight and height. While BMI is not always a reliable measure of health, the U.S. Centers for Disease Control and Prevention (CDC) consider a BMI of 25-29 “overweight” and a BMI of 30 or higher “obese.” (By the way, BMI is not an intuitive measurement… If you don’t know your own BMI and you want to, this handy web calculator will help.)
But does body fat matter for birth control? Good news first: it doesn’t matter for most methods, including the IUD, the implant, the shot, the ring, and condoms. These methods are all effective for women of any weight. Now the bad news: weight definitely matters for emergency contraceptive pills, and we don’t know for sure whether it matters for the commonly used pill, or for the patch.
So what’s the deal with overweight women using emergency contraception?
A study of over 3,445 women using different types of emergency contraceptive (EC) pills—Plan B and ella—found that women with a BMI over 25 had an increased risk of a pregnancy. The risk of an accidental pregnancy was greater with Plan B than with ella, and greater for women with any kind of EC pill when their BMI was 30 or over compared to under 30. So for women who have a BMI over 25 and are worried about an accidental pregnancy, a copper IUD may be the best EC choice. Copper IUDs are almost 100% effective as EC when they are placed within 5 days after unprotected sex.
What about the pill and the patch?
Nobody knows for sure how body fat may affect birth control pills or the patch, but there are a lot of theories. One has to do with the way our bodies break down medicines, also known as metabolism. Women who are overweight tend to have speedier metabolisms, so they break down all kinds of medications faster. A birth control pill taken by a woman with more body fat may be broken down faster than the same pill taken by a woman with less body fat—and the more rapid break down could make the pill less effective. Another theory is that overweight women have more blood in their bodies. In order for any medication to be effective, it must reach a certain concentration in the bloodstream. For women with excess weight and more blood, it may be more difficult for a medication to get to the right level in the bloodstream. But at this point these are just theories…
Why don’t we have an answer, science peeps?
This shouldn’t be such a hard question to answer from a scientific perspective, but unfortunately many studies of hormonal birth control have not included overweight women. Of the limited studies that have included women of all sizes, some have found an increased risk of accidental pregnancy among overweight women using birth control pills. But these studies weren’t perfect because they didn’t measure women’s weight when they started or find out how consistently they used the pill. One big review looked at all studies of hormonal birth control and found that four of seven showed a higher risk of accidental pregnancy in obese women using the pill or the patch.
So what birth control do doctors recommend for overweight women?
While there’s still a lot we don’t know about weight and birth control, we do know that accidental pregnancies can be especially dangerous for women with an unhealthy body weight. So using an effective method of birth control can be especially important for overweight women who don’t want to be pregnant—the implant and IUD are excellent choices. Of course, if you’re concerned about this issue, you should get a personalized recommendation from your health care provider.
*Note: BMI can be a great diagnostic tool in medicine and personal health monitoring, but it’s not always a reliable measure of health. If you’re interested in the details, Livestrong.com has a good article about it.