Some in vitro studies have shown genetic damage to mammalian cell lines exposed to high Hypericum extract concentrations. Others have shown impaired sperm motility, decreased sperm penetration of the egg, and denaturation of sperm DNA with exposure to very high concentrations of Hypericum extract.
Two controlled trials in mice demonstrated a significant reduction in birth weight of mice exposed to St. John's wort in the uterus, but no difference in neurobehavioral development or reproductive capacity. Another similar study in rats showed that female rats exposed to Hypericum prenatally required more time to learn a new maze task. The clinical relevance of this, however, is uncertain. One randomized, blinded trial on rats showed reduced size and reduced number of somites in rats exposed to hypericin, a substance found in St. John's wort.
The two human case reports of women self-treating with St. John's wort reported no ill effects to either mother or infant.
Given the limited data regarding the safety of St. John's wort during pregnancy and the wide availability of other treatment options, such as psychotherapy and prescription antidepressants, St. John's wort is not recommended for pregnant women at this time.
Holstege, Christopher P., et. al. “Toxicity and Drug Interactions Associated with Herbal Products: Ephedra and St. John's Wort.” Medical Clinics of North America 89.6 (2005): 1225-1257.