American scientists are warning pregnant women against taking herbal supplement ginkgo biloba because it can be highly toxic.

Researchers at Wayne State University in Detroit studied routine tests from 24 pregnant women. They discovered five of them had high levels of the toxin colchicine. They had all been taking the supplement.

Colchicine is found in a number of plants and is sometimes used to treat gout. Scientists say it can interfere with cell division and prove fatal at very high doses. It can also affect fetal growth.

Colchicine can cause nausea, vomiting, abdominal pain, and diarrhea. At its worse, it can damage the bone marrow causing severe anemia.

Gingko biloba is used to treat memory loss and for those suffering from Alzheimer's or circulatory problems. It is said to boost oxygen levels in the brain.

The Chinese have used extracts from the leaves of the ginkgo biloba tree for centuries to treat a variety of medical conditions.

Dr. Howard Petty who headed the study says they only examined one brand of supplement. He says pregnant women should always consult their physicians before taking any type of medicine, herbal or otherwise.

"Such supplements should be avoided by women who are pregnant or trying to conceive," said Petty in his report.

The report is published in this week's issue of New Scientist and in the journal Chemical Research in Toxicology.