The problem of green tea during pregnancy is that the EGCG molecules are structurally similar to a compound called methotrexate. Methotrexate is able to kill cancer cells by chemically bonding with an enzyme in the body called enzyme dihydrofolate reductase (DHFR). Healthy people have this enzyme also - it is part of what is called the folate pathway, which is the pathway, or steps, the body takes to transform nutrients like folate into something that can be used to support its normal functions.
But this chemical similarity means that the EGCG in green tea also binds with the enzyme DHFR, and when it does this, it inactivates this enzyme. When this enzyme is inactivated, the ability of the body to use folate is going to be affected. How much green tea is able to be consumed, or precisely how much folate absorption is affected, is unclear. Though the research article did say that drinking 2 cups of green tea a day can stop cancer cells (which is what methotrexate is targeting) from growing.
The good news on caffeine drank during pregnancy, from coffee and tea, is that a moderate amount is fine. Two studies, one by Danish scientists who interviewed more than 88,000 pregnant women, and the other by the Yale University School of Medicine, had similar findings on caffeine during pregnancy.
The concerns over caffeine were that it would lead to low birth weight or miscarriage. And this is still true of a very high daily intake of coffee. The Yale team found that drinking about 600mg of caffeine a day, which is about 6 cups of coffee, would reduce birth weight to levels that were clinically significant. The rate at which birth weight was reduced was established at being 28 grams per 100 mg, or 1 cup, of coffee per day. But they emphasized that this would not be significant for moderate caffeine consumption.
The Danish study found that drinking 8 cups or more of coffee per day (this would be about 16 cups or more of tea), would increase the chances of miscarriage, or stillbirth, by 60% compared to women who did not drink caffeine. They also found that moderate coffee or tea drinking did not pose significant risks. For those drinking half a cup to 3 cups of coffee a day, the risk of fetal death was 3% higher compared to non-caffeine drinkers. And for those drinking 4 to 7 cups of coffee a day, the risk increases to 33%. One cup of coffee equals about 2 cups of tea when comparing caffeine levels.
The recommended amount of coffee drunk is up to 3 cups daily, or 6 cups of tea, by the UK food agency.