Caffeine and Pregnancy: How Risky?

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A pregnant woman sips coffee at the office.

Several studies suggest a link between caffeine consumption and risk of miscarriage. But the cause and effect has never been clear: does caffeine increase a woman's risk of miscarrying, or do women who are already at low risk for miscarriage tend not to drink caffeine? (To wit: morning sickness, which is a sign of a healthy pregnancy, hardly makes you crave a cup of Joe.) At Kaiser Permanente Northern California's Division of Research, reproductive and perinatal epidemiologist Dr. De-Kun Li wanted to parse the association. He and colleagues recruited 1,063 women in early pregnancy, quizzed them about their caffeine intake and followed them to the end of their pregnancies. The researchers also checked to see, crucially, whether women had altered their caffeine-drinking habits after becoming pregnant — that way, scientists were able to control for the effect of morning sickness. TIME asks Li's expert opinion. (Click to hear the podcast.)

Q: How much caffeine is safe to drink during pregnancy?

A: What we found is that for women whose caffeine intake was more than 200 mg every day, the risk of miscarriage was double compared with women who did not have any caffeine during pregnancy. For those who consumed more than 200 mg a day, their miscarriage rate was about 25%. Below 200 mg, we also saw a slight increase in risk, [but] we were not able to make a more definitive assessment. Generally speaking, though, for [risks due to] environmental exposure in the context of miscarriage, there is rarely a threshold effect. Usually, the higher the dose, the bigger the effect.

I would think, given this study finding and other study findings before, the first choice for women should probably be to stop drinking caffeine entirely — at least for the first three to four months of pregnancy. But if you really have to drink, I think you should limit yourself to one regular coffee a day or less. Two hundred mg of caffeine is roughly about one-and-a-half to two [8 oz] cups of regular coffee. [In our study], most of the caffeine came from regular coffee. [It's also found in tea, hot chocolate and soda.] [If you drink] regular soda, 200 mg is roughly equivalent to five 12 oz cans.

There are two mechanisms that have been hypothesized up to this point, linking caffeine to miscarriage. First, we know that caffeine can directly, easily cross the placental barrier. While adults can usually metabolize caffeine, a fetus usually can't, particularly in the early stages of development. So caffeine has a direct effect on cells, membranes and tissues. Those kinds of things for adults are OK. That's why we actually drink caffeine — because it interacts with our cell receptors. But for a fetus, particularly at the early stages of development, it may interfere with development.

Second, it's been known for a long time that high doses of caffeine can have a vaso-constrictive effect [in adults], which means it can make blood vessels contract. If the effect is severe in the mother, hypothetically it can reduce blood flow to the placenta and to the fetus. It's totally possible that [caffeine could have an impact in later pregnancy as well]. There could be other [associated] outcomes, like low birth weight, preterm delivery, even birth defects, or [problems with] neurological development. Those are all biologically plausible effects. It's just that we haven't demonstrated them. They're harder to study.

The message here is probably the less, the better. I would think if you wanted to be safe, you should probably cut down or even stop caffeine-drinking throughout pregnancy.