It's that time again. Maggie is back at the largest science convention in the Western Hemisphere for four days of wall-to-wall awesomeness. Each day, she'll tell you about some of the cool things she learned watching scientists from all over the world talk about their work. Check the bottom of each post to find links to earlier posts in this series!

Each year, the American Association for the Advancement of Science holds a conference. Scientists from every discipline you can think of attend. They come from all over the world bearing fascinating studies they're dying to talk about, and Power Point presentations they'd probably rather I didn't critique. The result: The worst part about this conference (besides the aforementioned poorly done Power Points) is trying to choose which session you want to see. There's often as many as a dozen occupying the same time slot. Usually, three or four of those will strike me as something I MUST find out more about.

Friday morning, I picked a session that I hoped would provide some background and context on issues you and I are already talking about. Birth control—and, specifically, who should have access to it—has become a major issue in the current presidential campaign. Along with that has come a lot of confusion and misinformation about how birth control works, how effective it is, and what we know about its potential side effects. My first session of the day: Fifty Years of the Pill: Risk Reduction and Discovery of Benefits Beyond Contraception.

The first thing I learned: If you're taking an oral contraceptive, there's a good chance that you're doing it wrong.

Only 50% of the women who use the pill actually use it correctly—never missing a day and taking the pill at the same time of day every day. In fact, in 1998, University of Michigan nursing professor Deborah Oakley did a study with a small group of nursing students that found that even women who think they use the pill consistently, often don't. Before the study, Oakley asked the women how often they missed their birth control pills. They told her one out of every 10. Then, she gave them electronic packs that recorded actual usage. "The reality: Their pill missage rate was 2-3x what they thought it was," says Ronald Burkman, professor of obstetrics and gynecology at the Tufts School of Medicine.

Burkman was one of the speakers on the panel, and he told me that inconsistent usage affects both how well birth control works and how well we can study its side effects. That's because "inconsistent usage" doesn't just refer to forgetting to take pills or not taking them at the right time. Those are the things that can lead to unwanted pregnancies, but there's a bigger picture as well. Women don't use birth control in a monolithic way. For instance, in my own life, I've used 3-4 different kinds of hormonal birth control—all with different formulations and dosages. And even though I used the pill for the first time when I was 18, I've not been on a hormonal birth control consistently since then. For various reasons, women go on and off of this medication. And that makes it difficult to study the side effects of birth control in a really granular way.

In general, we know that birth control is pretty safe. Even the scary-sounding side effects, like heart attack, are incredibly rare. What's more, pregnancy is more dangerous. For example, even though birth control slightly increases the risk of heart attack, pregnancy (and particularly the period immediately following pregnancy) increases your risk far more. Burkman said that studies show somewhere between 2 and 14 cardiac events per 10,000 woman-years of birth control use. Meanwhile, for every 10,000 pregnancies, 20-50 women will have some kind of a cardiac event. (That's true for all the big risks of birth control, Burkman said. The absolute increase in risk is tiny, and you're safer with birth control than you are with pregnancy.)

We know this because of two different kinds of studies. One type of study compiles the medical information of thousands of women who use birth control, aggregating the amount of time they each used it. That's what is meant by "10,000 woman-years of use". There might be 200,000 women in the study, and between them, they've used the pill for 10,000 years.

The other type of study follows individual pill users over the course of their lives and records what happens to them. At this point, we have data following pill users over 39 years. Those studies actually show some interesting benefits to birth control use. Compared to women who never took the pill, the pill users had increased bone mass, fewer cases of endometriosis—when uterine cells start growing outside the uterus and cause a lot of pain—and fewer cases of pelvic inflammatory syndrome, bacterial infections in the reproductive system that can lead to scarring and infertility.

But, at the same time, there's really not much data that breaks out the risk analysis by type of birth control user. If one woman takes birth control for 2 years, and another takes it for 20, we don't really know much about how their relative risks differ. That's because 2 years of use (or 20 years of use) doesn't mean the same thing for one woman that it means for another. How do you know who has been using the pill consistently during that time and who hasn't? How do you compare women who've used two completely different types of pills? Different chemical formulations have different effects on the body.

During the Q&A portion of the session, a woman told the panel that she was concerned about what happened to women who used birth control for 15 years straight, starting in their mid-teens. "It can't be safe," she said. "I've personally known too many people with fertility problems and blood clots."*

The truth is that we have enough data to say she's wrong. We can look at the aggregate studies and see that birth control is safe, that it's safer than pregnancy. We can look at the longitudinal studies (the ones that follow women over the course of their lives) and see that birth control doesn't cause fertility problems. But, frustratingly, we can't give that woman the exact kind of data that she's looking for. We can't point to a study of 30-year-old women who have been on birth control since they were 15 and tell you what happened to them.

There's a possibility that that could be easier in the future, as more women use forms of birth control that are harder to use inconsistently—things like the monthly ring, the three-year implant, or the 5-10 year hormonal IUD. But these still aren't easy studies to set up, and you still can't compare a ring user to an implant user, and expect that to say something about women who use the pill. The data we have today is good—but it's not granular. We can speak about women who use the pill in general. But we can't tell you much about particular women.

*Personal observation of a friend group shouldn't be taken as a signal of what actual risks look like. Here's a good example of why: I'm about the same age as this woman was, roughly 30. Like her, most of my female friends have been on hormonal birth control of some sort at one time or another since their mid-to-late teens. I know nobody who's had a blood clot. Or any side-effects more serious than unpleasant emotional yuckiness. I know a small handful of women who have had fertility problems, but all of them also had irregular periods as teenagers, something that is often a sign of underlying, natural fertility problems.

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