Roughly 1 in 10 pregnant women in the U.S. expose their unborn baby to herbal products, according to a new study.

This finding is potentially concerning, researchers say, given that data on the safety of herbal use during pregnancy is lacking. Furthermore, the prevalence of exposure was highest in the first 3 months of pregnancy, a critical period of development.

"If we assume that our study sample was representative of the 4.2 million births each year in the United States, our findings project that 9.4 percent, or potentially 395,000 U.S. births annually, will involve exposure to at least one herbal product during pregnancy," lead author Dr. Cheryl S. Broussard, from the Centers for Disease Control and Prevention in Atlanta, told Reuters Health by email.

The findings, reported in the American Journal of Obstetrics and Gynecology, are based on data from 4,239 mothers in the National Birth Defects Prevention Study who delivered liveborn infants with no major birth defects from 1998 to 2004. The 10 study centers were located across the U.S.

Overall, 462 (10.9 percent) of the 4,239 mothers reported using herbal products in the 3 months before or at some point during pregnancy. The prevalence of herbal use anytime during pregnancy was 9.4 percent. The prevalence during pregnancy was highest (6.9 percent) during the first trimester.

Excluding the 86 mothers whose only use of herbs involved herbal teas, the prevalence of herb exposure before or during pregnancy was 8.9 percent.

The most commonly reported products used early in pregnancy were ginger, which has been shown to help ease morning sickness, without side effects to the unborn child, and ephedra, an herbal stimulant that was banned in the U.S. in 2004 after reports linked it to heart attacks, strokes and at least 155 deaths.

The products used most often throughout pregnancy were herbal teas and chamomile, which has also been shown to ease morning sickness.

Herb exposure was more prevalent in women older than 30 and with more than 12 years of education. Of the 10 states studied, Iowa had the lowest rate of herbal use (5.4 percent) and Utah had the highest (16.5 percent).

"The fact that use of herbal products was greatest during the first trimester raises concerns about fetal safety, because this is a critical period of fetal organ development," Broussard noted.

"Providers should inform patients that it would be prudent to err on the side of caution regarding use of herbal products just before and during pregnancy because little is known about their potential risks."