Rewire takes a look at the recent media storm around emergency contraception and weight and explains what readers need to know about the research.

Once again, politics have trumped science, and it’s women and girls who pay the price.

Recently, there has been a lot of discussion in the news about emergency contraception pills and whether they are effective for women over a certain weight. Below is Rewire‘s explanation of why this story came to light, and what women who may need emergency contraception now or the future should know.

Why is everyone talking about weight and emergency contraception right now?

A few weeks ago, a European manufacturer of emergency contraceptive pills identical to some of those on the market in the United States announced it was changing its labeling to add a caution that the product is less effective in women with a higher body mass index (BMI). This decision was based, at least in part, on the company’s own research, but there is other published research that has found the same thing. The media picked up the story and questioned whether the Food and Drug Administration (FDA) was going to ask or require U.S. manufacturers to make the same labeling change.

Remind me: What is emergency contraception?

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Emergency contraception is birth control that is used to prevent pregnancy after a couple has unprotected vaginal intercourse. In order to be effective, emergency contraception has to be used within 120 hours of unprotected sex.

Emergency contraceptive pills (ECPs) are taken orally and work by preventing ovulation and therefore pregnancy. There are two types of ECPs on the market in the United States. The first type is referred to as LNG pills because they rely on a synthetic version of the hormone progesterone called Levonorgestrel. LNG pills are sold under the brand name Plan-B One Step as well as the generic versions Next Choice One Dose and My Way. This is the type of ECP that is available over-the-counter to women of all ages in the United States. This is also the type for which the label was changed in Europe. The other type of ECP relies on a chemical called Ulipristal acetate (UPA) and is sold under the brand name Ella. This is not available over-the-counter.

It is also possible for a woman to have a copper intrauterine device (IUD) inserted into her uterus after unprotected sex, which can prevent pregnancy and also continue to act as a birth control method for up to ten years. Copper IUDs are more than 99 percent effective at preventing a pregnancy when inserted on an emergency basis. IUDs, however, can be very expensive and must be inserted by a health-care provider, which makes it difficult, if not impossible, for many women to access this method, especially within the 120-hour window.

What is the research on ECPs and weight?

In 2011, researchers analyzed two randomized controlled trials on the efficacy of LNG and UPA pills to try to understand why emergency contraception does not always work. They found timing to be the most important predictor of whether ECPs would work—how close a woman was to ovulation when she took the pills. However, they also found that ECPs were significantly less effective in women who were overweight or obese. Specifically, they found that women with a BMI of 30 or greater who used either kind of ECP became pregnant more than three times more often than women who were not obese.

The analysis found differences between the two types of pills. UPA pills were effective in women with a BMI of up to 35 but LNG pills lost efficacy at a much a lower BMI. In fact, pregnancy rates among women with a BMI of 26 or higher who took LNG pills were no different than would have been expected had they not used EC at all.

Body mass index is a way to determine obesity based on an individual’s height and weight. A BMI of under 25 is considered healthy, between 25 and 29 overweight, and 30 and over obese. As an example, a woman who is 5′ 6″ and 160 pounds would typically have a BMI around 26, whereas a woman who is 5′ 2″ and 160 pounds would have a BMI of over 30. The average weight for U.S. women is 166 pounds, and 35 percent of people in the United States are obese. Though BMI remains the most common tool used to measure obesity, there is an ongoing debate as to whether it is a reliable measure or a good indicator of overall health.

To put the research in terms of weight, which can be easier to understand, LNG pills seem to lose efficacy beginning at 165 pounds and become ineffective in women above 176 pounds.

Can’t women over a certain weight just take a higher dose?

As of now, there is no research to suggest that taking a higher dose of ECP pills would increase their efficacy, and it is not recommended that women do so.

Should women who are over a certain weight not use emergency contraception at all?

No. Health-care providers are still figuring out what the research really means and whether it should change what women do. Many think there isn’t enough research to recommend such a change yet. That said, it’s important that women know the research and discuss it with their provider, ideally before they need emergency contraception.

Women over 165 pounds who need emergency contraception may want to consider using UPA pills like Ella or getting a copper IUD inserted instead. As mentioned above, however, access to these methods, especially within the 120-hour window, can be difficult because they require a health-care provider either to write the prescription or to insert the IUD. They may also be cost prohibitive.

Women who cannot access these other methods quickly may want to considered using the LNG pills, as they may work and the side effects are minimal.

Will the FDA require label changes in the United States as well?

That remains to be seen. After the news of the label change in Europe, many people questioned whether the FDA would require a similar label change for Plan-B One Step and its generic equivalents. The FDA has told media outlets that it is reviewing the information and research and will make that determination in the near future.

Does this research apply to birth control pills as well?

Though some monthly oral contraceptives do rely on LNG, there is no research to suggest that birth control pills are ineffective in women over a certain weight or BMI. However, there is some research to suggest that the contraceptive patch may be ineffective in women over a certain weight; women using or considering the patch may want to talk to their health-care provider about that.