A large number of pregnant women and mothers breast-feeding their babies are deficient in necessary iodine to produce a hormone essential for natural brain development in a baby or infant.

Such women should consider a daily supplement of iodine, the American Academy of Pediatrics says.

Most iodine in the human diet comes from iodized salt, but an increase in consumption of processed foods -- in which the salt has not been iodized -- leads to increasing iodine deficiency with about a third of pregnant U.S. women.

Pregnant women and those that are breast-feeding should use a supplement with a minimum of 150 micrograms of iodide and should include table salt that has been iodized in their diet, researchers said in article in the journal Pediatrics.

"This is the first time that the American Academy of Pediatrics has issued a statement on iodine," says Dr. Jerome Paulson, chair of the academy's Council on Environmental Health.

While a significant proportion of pregnant U.S. women have been found to be iodine deficient, the journal article points out, only around 15 percent of such women and women breast feeding infants are taking advantage of supplements.

Iodine supplements include sodium iodide and potassium iodide, with the potassium supplement being preferred.

While a severe deficiency in iodine has been linked with stunted mental and physical growth, pediatricians say, even slight deficiencies can have an effect on brain development and functioning.

It is unclear why more women aren't taking advantage of readily available supplements, Paulson said.

"It may be that most people don't appreciate the importance of adequate iodine in the diet for normal fetal development and that the women with marginal levels have no indication of their iodine status," he said, noting the lack of a display of symptoms in iodine deficiency.

Because around half of pregnancies in the United States are unplanned and women in the early period of the condition may not realize they're pregnant, it is important to be aware of the issue of iodine deficiency, he said.

In addition, women who are vegan or who do not eat fish should consider being tested for an iodine deficiency if they are considering having children, the academy's council says.