A new NIH study on pregnancy does not prove caffeine causes miscarriages.

Don’t Believe the Scary Warnings That Caffeine Before Pregnancy Can Increase Miscarriage Risk

Don’t Believe the Scary Warnings That Caffeine Before Pregnancy Can Increase Miscarriage Risk

What women really think.
April 1 2016 5:58 AM

Wake Up and Smell the Coffee

Don’t be fooled by the scary reports that drinking caffeine before conception causes miscarriages.

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It’s tough to know whether the caffeine consumed by women in the new study was, in fact, responsible for their increase in miscarriage risk.

Jewell/Flickr CC

Last week on the Today show, Matt Lauer told millions of Americans about “a new warning for anyone who’s trying to have a baby.” He described a study recently published by researchers at the National Institutes of Health that, according to Lauer, showed that “consuming caffeinated drinks, even before pregnancy, can raise the risk of miscarriage by 74 percent.”

Wow. That’s scary enough to make a caffeinated mom-to-be even more jittery. Problem is, what Lauer said isn’t true. He’s right that a study was published on the relationship between caffeine consumption and miscarriage, adding to the body of conflicting research on the issue. But as I’ll explain, there are reasons to question whether caffeine actually causes miscarriage or whether it’s associated with it for unrelated reasons. When Today medical contributor Natalie Azar tried to explain to Lauer that a causal interpretation of the findings—what he had just described on national TV—was in fact inaccurate, he quickly changed the subject.

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Also rather important: Regardless of whether the link the researchers found is causally related, the study only found ties between miscarriage and the consumption of more than two caffeinated drinks a day. That’s vastly different from Lauer’s fearmongering implication that, OMG, any amount of caffeine for anyone who wants to have a baby is dangerous! When I spoke with a co-author of the NIH study, Germaine Buck Louis, an epidemiologist at the Eunice Kennedy Shriver National Institute of Child Health and Human Development, she told me, “You could look at this as good news, because our findings don’t say that you can’t have caffeinated beverages.”

Buck Louis’ study, published online March 22 in the journal Fertility and Sterility, was well done. It tracked women and their partners as they tried to get pregnant and through the earliest stages of their pregnancies, which means the researchers were able to record very early miscarriages. (Many previous studies on the relationship between caffeine and miscarriages started tracking groups of pregnant women when they were 8 to 10 weeks along, which means they omitted women who had earlier losses.) It is also among the first studies to investigate how the lifestyle choices women and men make prior to conception relate to miscarriage risk. It found that women who drank more than two coffees, teas, or sodas a day before conception were 1.75 times as likely to miscarry, and that those who drank this daily amount after conception were 3.05 times as likely to miscarry, compared with women who drank less. It also found that the pregnancies conceived by men who drank this amount prior to conception were 1.73 times as likely to miscarry compared with pregnancies conceived by men who drank less. Another major study strength was that the researchers had participants fill out a daily diary of what they ate and drank, rather than collect this information in after-the-fact surveys, which are notoriously inaccurate.

So it’s a good study, but nevertheless, it simply can’t tell us that drinking lots of caffeine increases miscarriage risk. That’s in part because there are so many factors at play in these kinds of observational studies, which study how the circumstances of people’s lives, including their lifestyle choices, affect them down the line. People often make choices that cluster together and can be difficult to tease apart, even with the help of statistics, and that makes it hard to prove that one particular choice is causing one particular outcome. Here’s a thought experiment: Let’s say that people who eat lots of quinoa also frequent the gym and regularly wear sunscreen. (I’m making this up, but you know I’m right.) So if you studied how quinoa consumption relates to skin cancer risk, you would likely find an inverse association, and Matt Lauer would tell his viewers that quinoa prevents skin cancer. But it’s far more plausible that the quinoa-eaters’ regular sunscreen use drives the association and that quinoa actually has nothing to do with skin cancer.

Along these lines, pregnant women who drink coffee or tea or soda often make other lifestyle choices that are distinct from the choices made by women who don’t. A number of studies have shown that women who consume a lot of caffeine while expecting are also more likely than other pregnant women to drink alcohol and to smoke—and alcohol and cigarettes are, of course, also tied to miscarriage risk. Which vice is actually causing miscarriages if they all tend to coexist? Buck Louis and her colleagues made statistical adjustments to try to control for the effects of these known “confounding factors,” as they’re called. They controlled for age (pregnant women who drink caffeine are usually older, and of course older women are more likely to miscarry), BMI (also linked to miscarriage risk), alcohol consumption, and smoking, among other things, but these statistical adjustments are a rather imprecise science. And to control for all confounders, researchers have to know what all the confounders are in advance. There may be other choices made by caffeine-drinking pregnant women that raise their miscarriage risk but which no one yet knows about. If so, these could influence findings but be impossible to detect. (Going back to our quinoa example, if you didn’t know quinoa eaters tended to wear sunscreen, you’d never figure out that their sunscreen use was the real driver of the inverse link between quinoa and skin cancer.)

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To sum up my belabored point, it’s tough to know whether the caffeine consumed by women in the new study was, in fact, responsible for their increase in miscarriage risk, or whether their other choices or characteristics were. Fun fact: The new study found that pregnancies conceived by men who drank more than two caffeinated drinks a day after conception were 1.88 times as likely to miscarry compared with pregnancies conceived by men who drank less after conception. Does this mean that the caffeine from Dad’s coffee jumps from his intestines into his partner’s womb and causes it to miscarry? Of course not. Clearly, there is something else driving this association, although it’s unclear what—and that’s precisely my point. The real meaning of an association can be tough to figure out. I’m not the only one who thinks this, by the way. In 2010, researchers at the University of Oklahoma Health Sciences Center and Harvard Medical School conducted an evidence review on the effects of caffeine on pregnancy loss in which they assessed all the research that had been published on the topic since 2000. Some studies they analyzed found an association while others didn’t. Their conclusion: “[T]he evidence for an effect of caffeine on reproductive health and fetal development is limited by the inability to rule out plausible alternative explanations for the observed associations.”  

The other problem worth noting is that “there is a huge challenge in accurately measuring caffeine,” says David Savitz, an epidemiologist and OB-GYN at Brown University. Participants in Buck Louis’ study were asked to record how many caffeinated beverages they drank each day, but for the sake of simplicity, they were told to count a cup of coffee, a cup of tea, and a can of caffeinated soda as equivalent—they all qualified as “one caffeinated drink.” Yet coffee, tea, and soda provide vastly different amounts of caffeine. A can of Dr. Pepper contains about 40 milligrams of the chemical, whereas a cup of regular coffee has 140 milligrams. And as Buck Louis added when we spoke, “we know that depending on how you brew the coffee, how long the water interchanges with the bean and a whole host of other things, there’s tremendous variation in the milligrams as well.” The upshot is that men and women in the study who drank, say, three cans of Dr. Pepper each day (about 120 milligrams of caffeine) were lumped together with people having three cups of strong coffee a day (upward of 500 milligrams of caffeine)—making it hard to figure out how caffeine intake during pregnancy exactly relates to miscarriage risk.

And who knows—maybe caffeine’s not really the culprit. One study found that drinking more than three cups’ worth of caffeinated drinks a day was associated with no rise in miscarriage risk but that, bizarrely, drinking the same amount of decaffeinated coffee was associated with a risk 2.4 times higher. Confused yet?

Given all the uncertainty, one might wonder why the message that’s so often heard is that pregnant women should stop drinking caffeine. (In fact, that’s the official recommendation of the uber-conservative American Pregnancy Association.) According to Princeton sociologist Elizabeth Mitchell Armstrong, author of Conceiving Risk, Bearing Responsibility: Fetal Alcohol Syndrome and the Diagnosis of Moral Disorder, this message may have more to do with cultural norms and cognitive biases than scientific evidence. “We expect mothers to be completely self-sacrificing for the sake of their children,” she explains. “This performance of maternal sacrifice during pregnancy—whether in the form of giving up caffeine, or alcohol, or some other form of abnegation—is expected of pregnant women today not only as a route to better fetal health, but as a means of demonstrating their moral fitness to become mothers.” Remember the controversial new Centers for Disease Control and Prevention campaign that discourages young women from drinking alcohol if they are not taking birth control? That seems to fit this pattern as well.

The bottom line: There is zero evidence to suggest that women (or men, for that matter) need to give up caffeine before or while they’re expecting. If caffeine and miscarriages are causally related—and that’s still a big if—then the risk seems to apply only to women who drink more than two caffeinated drinks per day. If you want to be extra cautious while you’re expecting or trying to conceive, sure, forgo some of the caffeine you might normally have. But don’t spend too much time worrying about it, either, because—wouldn’t you know it—stress is associated with pregnancy problems, too. Of course it is.

Melinda Wenner Moyer is a science writer based in Cold Spring, New York, and is Slate’s parenting advice columnist. Follow her on Twitter.