What specific danger do certain herbal compounds pose to a developing fetus during pregnancy?

Herbs can possess properties that stimulate the uterus, potentially leading to preterm labor or miscarriage

The caution surrounding herbal teas is significantly intensified for pregnant individuals because the developing fetus is highly susceptible to substances crossing the placenta. Many herbs, even those safe for the general population, lack sufficient safety studies in this vulnerable group. A primary concern is that certain botanical compounds possess uterotonic properties, meaning they can cause uterine contractions. If these contractions occur prematurely, they raise the risk of adverse outcomes such as preterm labor or, tragically, miscarriage. Furthermore, compounds can also transfer into breast milk, impacting the nursing infant, but the direct risk to the fetus involves these uterine stimulation properties.