Researchers have found the potentially harmful compound colchicine in the placental blood of women taking the popular herbal medicine Gingko biloba.
Previous research has indicated that colchicine can harm a woman's developing foetus, the American Chemical Society reports this week.
According to scientists Shahriar Mobashery and Howard Petty, who led the study at Wayne State University in Detroit the colchicine might have come from commercially available Gingko biloba (gingko) supplements taken by some of the women in their study.
Colchicine, an alkaloid, has beneficial effects as an anti-inflammatory drug that can prevent cell growth.
But at high doses it can be fatal, the researchers warn.
The substance, not normally found in humans, is present in nearly 200 species of plants.
The researchers caution that they have no evidence that gingko taken during pregnancy causes birth defects, suggesting only that the potential for problems exists and that expectant mothers should avoid taking the substance as they should avoid other risks, like smoking or drinking alcohol.
Petty suggested the situation may be analogous to a pregnant woman's drinking coffee: while the caffeine from a cup of coffee is quickly cleared from the mother?s system, it can build up in the womb.
Mobashery added that further study would be needed to prove a correlation between gingko and complications during a pregnancy.
Their study used placental blood samples taken from 24 new mothers in the Detroit area.
Measurements confirmed that the five women who regularly took gingko had colchicine levels ranging from 49 to 763 micrograms per litre of blood, while the remaining 19, which included a few vegetarians, had less than two micrograms of colchicine in their placental blood, Petty said.
The average colchicine content was approximately 26 micrograms per tablet of store-bought Gingko biloba, although the researchers declined to say which company's brand was measured.
Herbal medicines are treated as food items by the U.S. Food and Drug Administration and therefore not analysed as drugs.
Full findings will be published in the September issue of Chemical Research in Toxicology, a peer-reviewed journal of the American Chemical Society.