Although tea is generally considered to be a healthy drink, and has preventative effects against many types of cancers, these same anti-cancer properties are what make it so dangerous during pregnancy.
Folate is a critical vitamin that prevents neural tube defects. Green tea can be problematic during pregnancy due to its ability to lower folic acid bioavailability. Black and green tea's caffeine content also contributes to an increased risk of miscarriage.
Green and black tea can lower the maximum blood concentration of folic acid in humans. A randomized cross-over study examined the effects of folic acid supplements with water, green or black tea. Subjects ingested 0.4 mg folic acid supplements with water, green or black tea (0.3 g extract/250 ml) or 5 mg folic acid supplements with water or green tea (0.3 g extract/250 ml). With a 0.4 mg folic acid dose, green and black tea reduced the mean maximum concentration of serum folate by 39.2 per cent and 38.6 per cent, respectively. At the 5 mg folic acid dose, the mean maximum concentration of serum folate was reduced by 27.4 per cent. Thus, even low concentrations of green and black tea can lower the bioavailability of folic acid.
Green tea contains the anti-cancer catechin (−)-epigallocatechin gallate (EGCG). EGCG inhibits dihydrofolate reductase (DHFR), the enzyme that coverts folic acid to the active tetrahydrofolate form, at concentrations found in the blood of tea drinkers. Tea catechins can also inhibit intestinal folic acid absorption. Green tea contains more EGCG than oolong tea, and much more than black tea.
A study that examined the link between green tea, oolong tea, and folate levels of pregnant Japanese women showed that those who drank 100-130 mL of tea or more per day had significantly lower serum folate levels compared to others. The study adjusted for confounding variables including dietary folate intake and use of folic acid supplements or multivitamins.
Many, but not all, epidemiological studies also show a link between drinking tea and neural tube defects (NTDs). Data from the population-based Atlanta Birth Defects Case-Control Study shows that maternal tea consumption during the periconceptional period (3 months before through the first trimester of pregnancy) increases the risk for spina bifida. The risk was highest for mothers drinking 3 or more cups of tea per day, but was also quite high for those who drank 1 to 2 cups per day.
A Chinese study also examined the link between tea and NTDs and included 487 cases and 688 controls. Women who drank tea daily during the periconceptional period had a 3-fold increased risk of having an NTD-affected pregnancy, compared to women who did not drink tea during this period. The link was present for anencephaly, spina bifida, and encephalocele. The elevated risk with daily tea drinking remained after adjusting for several confounders including maternal age, and periconceptional folic acid supplementation.
Caffeine, and coffee, have been linked to an increased risk of miscarriage.
Although the caffeine content of green tea is low compared to coffee's, it can range from 24-45 mg per average 237mL cup, and the amount in black tea can range from 14-70 mg per 237 mL. A recent study examined the effect of caffeine, derived mainly from Japanese and Chinese tea (green teas), during pregnancy. The study included 858 Japanese women who each delivered one infant. The main sources of caffeine were Japanese and Chinese tea (73.5 per cent), coffee (14.3 per cent), and black tea (6.6 per cent). Maternal total caffeine intake during pregnancy was significantly associated with an increased risk of preterm birth. Higher Japanese and Chinese tea consumption was associated with an increased risk of preterm birth.
A recent study that included 5132 Danish women sought to examine if caffeine and caffeinated beverage consumption during preconception and early pregnancy is associated with the risk of spontaneous abortion.
Women who consumed high amounts of caffeine during early pregnancy had a higher risk of spontaneous abortion. Drinking less than 2 but more than 0 servings of coffee, black tea, and herbal/green tea during early pregnancy was linked to an increased risk of spontaneous abortion compared with 0 servings of these beverages. The risk peaked at 100-199 mg of caffeine per day, the amount in 1 to 2 cups of coffee, or 1.5 to a bit less than 3 cups of black tea per day.
Fetuses eliminate caffeine very slowly, so maternal caffeine consumption could increase fetal caffeine to very high levels. Caffeine causes hormonal changes and this could affect the risk of spontaneous abortion, and caffeine affects blood flow to the fetus.
Women should avoid green, oolong, and black tea if they are pregnant, or trying to become pregnant, because of tea's effects on folate levels, and the potentially dangerous effects of caffeine found in tea.