Pregnant women have been advised to keep their consumption of coffee and other caffeinated beverages (tea, sodas, cocoa, energy drinks), and chocolate containing foods to a minimum for decades. Currently the American College of Obstetrics recommends that women consume less than 200 mg of caffeine (from any source) per day during pregnancy. This is less than 2 cups of regular coffee or 4 cups of regular black tea. But a recent study's findings suggest that the levels should be kept much lower.
The study of 941 mother/baby pairs in Ireland found that each 100 mg increase of caffeine per day was associated with a lower birth weight, shorter length of pregnancy (gestational age), shorter birth length of the baby, and smaller head circumference of the baby at birth. The strongest associations between those who consumed the most caffeine (when compared to those who consumed the least) was with lower birth weight. The researchers think this occurs because caffeine crosses the placenta easily, but during pregnancy there is a slowed metabolism of caffeine (so it takes longer to get it out of the body). Similar results have been found in other recent studies. [On the other hand, for not pregnant women - coffee and tea are linked to all sorts of health benefits - here, here.]
From Medical Xpress: Caffeinated beverages during pregnancy linked to lower birth weight babies
A team of researchers at University College, Dublin, has found a link between caffeinated beverage consumption during pregnancy and low birthregularweight. In their paper published in the American Journal of Clinical Nutrition, the group describes their study of 941 mother and baby pairs born in Ireland and how they fared when the mothers consumed caffeine.
Prior research has shown that consuming caffeine while pregnant can lead to problems with a developing baby, but until now, researchers believed there was a safe level. Women have been told if they drink fewer than two cups of coffee or three cups of tea per day, there will be no adverse impact on their baby—the American College of Obstetrics has suggested that woman consume less than 200 milligrams of caffeine per day, regardless of its source. The researchers in this new report suggest that such information is wrong. In their study, they showed that drinking even safe levels of such beverages can cause lower birth weights—it can also reduce the size of the newborn's head and the gestational age at birth.
In their study, the researchers looked at data associated with 941 mother/baby pairs and their caffeine consumption habits during pregnancy. In so doing, they found a link between lower than normal birth weight and the amount of caffeine consumed. More specifically, they found that for each additional 100 mg of caffeine a woman drank per day during the first trimester, there was a reduction in birth weight by 2.5 ounces. They also found that those women who consumed the most caffeine had babies who were on average six ounces lighter than were babies born to women drinking the least amount of caffeine. Notably, the numbers contradict the so-called safe levels of caffeine consumption that have previously been reported. The researchers suggest that pregnant women reduce their intake of caffeine as much as possible during pregnancy.
It is not known how caffeine can cause low birth weights, but the researchers suggest it is likely due to it restricting blood flow in the placenta.