Pregnancy is one of the few situations when the use of herbal products can become complicated.
In many cases, there is little reliable human data about the safety of using herbal products during pregnancy or lactation. There is historical wisdom about the use of herbs during pregnancy, but I'm not sure this would apply to the concentrated, standardized products used today. And for ethical reasons, it is doubtful that any human trials will be conducted to test the safety of herbals during this crucial time. Most of the data collected about herbs and pregnancy is from animal tests, in vitro experiments, historical warnings and case reports. While these sources will not give a definitive answer, they will provide a rationale to exercise caution.