WEDNESDAY, Feb. 25 (HealthDay News) -- Most prenatal vitamins marketed in the United States don't contain as much iodine as is stated on the label, researchers report.

The variance is troubling, they say, since iodine is critically important to the health of a developing fetus.

In a letter appearing in the Feb. 26 issue of the New England Journal of Medicine, the team also noted that the actual levels of iodine found in the supplements were typically below those recommended by the American Thyroid Association (ATA).

"Iodine nutrition is critically important in pregnancy," explained Dr. Elizabeth Pearce, co-author of the letter and an assistant professor of medicine at Boston University Medical Center. "Women who are deficient in pregnancy have children often with lower IQs or neurocognitive delays. Iodine deficiency is the leading cause of preventable mental retardation in the world."

But, based on the new findings, "it seems that an ideal prenatal vitamin, in terms of iodine, does not exist," she said. "About half of them have iodine that's derived from kelp and that's very variable."

Through an Internet search, the authors found 127 nonprescription and 96 prescription prenatal vitamins currently available in the United States.

Product labeling on 114 products (87 nonprescription and 27 prescription) claimed that the vitamins contained iodine. According to the labeling, 101 (89 percent) of these products contained at least 150 micrograms of iodine in a daily dose.

Sixty-seven vitamins contained iodine from potassium iodide, 42 from kelp, and five from some other source.

"Products containing iodine from potassium iodide tended to be more consistent, [but] 150 micrograms of potassium iodide is not the same as 150 micrograms of iodine," Pearce stressed. "If you really want people to get what the American Thyroid Association [ATA] has recommended -- which is 150 micrograms [of iodine] a day in a supplement -- there isn't one, but we would prefer products made from potassium iodide."