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Pregnant women face a lot of dos and don'ts when it comes to food and drink, as in other areas of life. Working out where to draw the line is not always easy - though having a good head for statistics can help. "When I first found out I was pregnant, I really wanted to have a cup of coffee. It was first thing in the morning. And then I thought all of a sudden, 'Oh my gosh - am I even allowed to have one cup of coffee?'" recalls Emily Oster, an associate professor of economics at the University of Chicago. She turned to the internet and found, not surprisingly, that there was no consensus. Then she found that even books disagreed... and her doctor didn't always agree with the books. Some writers said pregnant women should avoid coffee completely. Others advised drinking no more than two cups. Yet others drew the limit at three. Continue reading the main story �Start Quote When you look at six or eight cups of coffee, there is some more evidence that that might be risky� Emily Oster "I've read books that said six. And so I felt like there must be an answer to this," says Oster. "The answer isn't both zero and six. Surely there is an actual number in here, and I wanted to try to understand both why there's so much disagreement but also really what is the right decision." Using her statistical training, Oster decided to review the medical literature herself. It is fine to have two cups a day, she concluded. But she describes herself as "more of a two-to-four cups a day coffee lady" - and at this level, she says, the evidence appears to be mixed. "Early on I felt terrible, and I was not really able to have any coffee which was very sad for me, but once I got into the second and third trimester and I was feeling better, I often had three cups a day and I felt comfortable with that. "When you look at more - at six or eight cups of coffee - there is some more evidence that that might be risky." Oster, now the proud mother of a healthy two-year-old girl, has gathered together her work in a book, Expecting Better. She hopes that where the evidence is mixed, readers can consider the facts and make their own decision, based on what they are personally comfortable with. It is difficult to draw firm conclusions, she says, because most of the studies are not randomised - it wouldn't be fair to divide pregnant women under study into two random groups and ask one group to drink coffee and the other to drink none. Consequently, Oster says, the people involved in the studies differ in many ways that could affect the course of their pregnancies, not just in their coffee-drinking habits. "The big issue is that caffeine consumption correlates very strongly with how nauseous you are. A lot of pregnant women are very sick, especially early on - the women who are sicker tend to drink less coffee. "But we know that being sick is a sign of a healthy pregnancy. And so when we see that women who drink less coffee also have more successful pregnancies we don't really know if that's just about coffee or whether it's really a confounding factor from this nausea." But coffee was just one item on a long list of forbidden, or semi-forbidden, items that Oster wanted to investigate. Alcohol for example. Some health services, like the National Health Service in England, recommend that women avoid alcohol altogether in pregnancy, but Oster says she decided that on the available evidence, she felt comfortable having three glasses of wine - in total - in the first trimester, and then half a glass three or four times a week in the second and third trimester. "One thing that comes out very quickly, which is very important to emphasise, is that heavy excessive drinking in pregnancy is very dangerous. That's something you see very clearly in the data.," Oster says. And while the risk of damage might be lower the less alcohol the pregnant mother drinks, that doesn't mean, Muthu says, that the magnitude of the damage will be lower. "The consequences could be just as bad as with higher levels of intake, and could result in permanent and severe physical and mental disability," he says. He adds: "There are additional difficulties around defining from the evidence what a 'low intake' would be in terms of units of alcohol, and how this might be interpreted by different people in practice."