During pregnancy, pregnant women should limit how much green tea they drink and be careful about taking green tea supplements. Green tea is full of antioxidants and is good for your teeth, blood sugar, cholesterol, and weight loss, among other things. But when scientists looked at the active part of green tea, called epigallocatechin, or EGCG for short, they found that it may change how the body uses folate. Folate is important for pregnant women because it keeps babies from being born with problems in the brain or spinal cord.
The problem with drinking green tea while pregnant is that the molecules of EGCG look like the molecules of a drug called methotrexate. Methotrexate kills cancer cells by forming a chemical bond with an enzyme called the enzyme dihydrofolate reductase in the body (DHFR). This enzyme is also found in healthy people. It is part of what is called the folate pathway, which is a series of steps the body takes to turn nutrients like folate into something it can use to keep its normal functions going.
But because of this chemical similarity, the EGCG in green tea also binds to the enzyme DHFR, which stops the enzyme from working. When this enzyme is turned off, the body won't be able to use folate as well as it could. It's not clear how much green tea can be drunk or exactly how much folate absorption is changed. Even so, the research article did say that drinking two cups of green tea a day can stop cancer cells from growing, which is what methotrexate is supposed to do.
The good news about drinking coffee or tea with caffeine while pregnant is that a moderate amount is fine. Two studies, one by Danish scientists who talked to more than 88,000 pregnant women and the other by the Yale University School of Medicine, came to similar conclusions about caffeine during pregnancy.
People worried that caffeine would make babies have a low birth weight or cause them to die before they were born. And this is still true if you drink a lot of coffee every day. The Yale team found that drinking about 600mg of caffeine a day, which is about 6 cups of coffee, would lower birth weight to levels that were clinically significant. The rate of birth weight loss was found to be 28 grams per 100 mg of coffee or one cup of coffee per day. But they stressed that this wouldn't be a big deal for people who drink moderate amounts of caffeine.
The Danish study found that women who drank 8 or more cups of coffee per day (or 16 or more cups of tea) were 60% more likely to have a miscarriage or stillbirth than women who didn't drink caffeine. They also found that drinking moderate amounts of coffee or tea did not pose any major health risks. People who drank between half a cup and three cups of coffee a day were 3 percent more likely to lose a fetus than people who didn't drink caffeine. And the risk goes up to 33% for people who drink 4–7 cups of coffee a day. When it comes to caffeine, one cup of coffee is about the same as two cups of tea. The UK food agency recommends drinking up to 3 cups of coffee or 6 cups of tea each day.
References:
http://www.nutraingredients-usa.com/news/ng.asp?id=58807
http://www.nutraingredients-usa.com/news/ng.asp?id=24747
http://www.foodnavigator.com/news/ng.asp?id=63174
