US health researchers said Thursday that they have documented lead poisoning risks among pregnant women who took Ayurvedic medicine and issued a new warning on the safety of traditional pills.

New York City health authorities probed six cases since last year of women -- all but one born in India -- found to be at high risk of lead poisoning due to Ayurvedic medicine, the US Centers for Disease Control and Prevention said.

Ayurveda, or long-life science, is a system of traditional medicine developed over thousands of years in India.

The pills -- none made in the United States -- are occasionally contaminated during manufacturing but some are "rasa shastra," the Indian practice of intentionally adding metals, minerals or gems to medicine.

The medicines are advertised as helping with pregnancy and at least one boasted that it would boost the chances of having a boy instead of a girl, said the report by the US government health organization.

While the six women have not shown symptoms, authorities found them to be at high risk of lead poisoning, which can damage the brain, kidneys and nervous and reproductive systems.

"Pregnant women present a unique concern, because lead exposure can adversely affect the health of both mother and child. Fetal lead exposure increases the risks for low birth weight, developmental delay, reduced intelligence and behavioral problems," it said.

The products contained up to 2.4 percent lead and some also contained mercury and arsenic, which are also considered dangerous for consumption.

The US Food and Drug Administration warned in 2008 to use caution when taking Ayurvedic medicines -- especially those sold over the Internet -- as they are generally not approved by regulators.

The researchers at the Centers for Disease Control and Prevention said that health care providers should ask patients about their use of foreign-made medicine and consider administering tests for metal exposure.

"The cases of lead poisoning among the six pregnant women underscore the importance of risk assessment for lead exposure and blood lead testing in at-risk populations," the report said.