For the same reason aspirin should be avoided in pregnancy, chamomile has such powerful anti-inflammatory properties that regular consumption may result in a serious fetal heart problem—premature constriction of the fetal ductus arteriosus, which allows the fetus to “breathe” in the womb.
For more on chamomile tea, see Red Tea, Honeybush, & Chamomile, and The Healthiest Herbal Tea. For other cautionary pregnancy tales, see What About the Caffeine?; Is Licorice Good For You?; Iron During Pregnancy; Maternal Mercury Levels; Pregnant Vegans at Risk for Iodine Deficiency; American Vegans Placing Babies at Risk; and Chicken Consumption and the Feminization of Male Genitalia. Another common herbal tea that may have a potential downside is yerba maté (Update on Yerba Maté). The best beverage during pregnancy is water.