The truth is, there are a lot of other factors that play an important role in the effectiveness of birth control besides weight, according to Alison Edelman, MD , at Oregon Health & Science University, one of the authors of the review. "If obesity is a factor, it's likely to be small and masked by other issues like [making sure you take the pill at the right time every day]," she says. So what does all this have to do with emergency contraception? Basically, the morning-after pill is a massive dose of birth control, and it acts on the same hormones as the daily pill does. Plan B specifically, which is the most widely used oral emergency contraception option out there, provides a large dose of levonorgestrel. That's a manufactured hormone that suppresses the surge of luteinizing hormone (LH) which, in a normal menstrual cycle, signals the ovary to release an egg. "You get this hormone that starts rising really rapidly," Dr. Edelman says, "and with levonorgestrel, you have to be taking it before [the hormone] reaches the peak for it to work." Whenever you take Plan B, no matter your weight, you want to take it as soon after you have sex as possible. That's partly because if you take it after that LH peak, it won't be able to stop ovulation and it'll fail. Back in that much-talked about 2011 study, researchers at the University of Edinburgh looked at what might make those failures more likely, including weight. The researchers analyzed data from two trials, which included information for nearly 4,000 women. Results showed that women who had sex right before ovulating, who had more unprotected sex, and unprotected sex right after using emergency contraception were more likely to become pregnant. And, as we mentioned, the researchers found that women with higher weights were more likely to have a failure than those with "normal" BMIs — especially if they had used Plan B, and not one of the other methods of emergency contraception. The thing is, though, that the study didn't really prove why or how much of an effect weight has on Plan B's efficacy. To start answering those questions, Dr. Edelman and her team recently gave 10 women levonorgestrel and measured the amount of the drug that actually made it into their systems. Their results, published in Contraception this July, show that women with BMIs considered "obese" only get about half as much of the drug in their blood compared to women with normal BMIs. Then, during their next menstrual cycle, the obese group got a double dose of levonorgestrel. That bumped their numbers up to match participants with normal BMIs. This was just a small pilot study and didn't measure the drug's effects on ovulation, which is what really matters in terms of preventing pregnancy, says Dr. Edelman. But it definitely suggests that, for women with higher BMIs, one dose of Plan B may not be enough to actually affect ovulation. Still, that doesn't mean that heavier women should be taking double doses of Plan B. For one thing, it's expensive. A single dose costs $50 and isn't always covered by insurance. Generic versions of Plan B do exist and are more likely to be covered by insurance, but are also somewhat expensive (around $40). But perhaps more importantly, there's even less research out there about the efficacy of double doses than single doses. In other words, we just don't know what happens if you take twice as much Plan B. Dr. Edelman says that the very best emergency contraception option for women of all weights is actually having a copper IUD placed shortly after unprotected sex. This option is even more effective than Plan B, but going through the hoops of getting a same-day appointment and then, you know, having an IUD placed (which for some women can cause cramping), is a lot to ask of women who are already stressed about a potential unintended pregnancy. In an ideal world, accidents wouldn't happen, and everyone would have no-fail birth control, but we know that's not always possible. In the meantime, we have to accept Plan B as the imperfect option it is. And we need to do more research to figure out the best back-up options for everyone.