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A study published Wednesday says some women, especially thin ones, could increase their chances of having bigger, healthier babies by taking zinc supplements during pregnancy.

Dr. Robert L. Goldenberg, chairman of the Department of Obstetrics and Gynecology at the Uni-versity of Alabama-Birmingham, led a team of researchers who studied 580 poor but healthy pregnant black women. The findings appear in the Journal of the American Medical Association."Our data strongly suggest that zinc supplementation improves pregnancy outcome in at least some pregnant women," Gol-den-berg said in a telephone interview. "It looks like it's very cheap, it's safe, and now I think we've proved that it's important."

Most supplements for pregnant women already include 15 to 25 milligrams of zinc, but only about half the women prescribed the pills take them, Goldenberg said. Zinc also is found in many foods, including seafood, meat, nuts and milk.

The study of women receiving prenatal care at four Jefferson County Health Department clinics in Alabama compared 294 women who took daily 25 milligram zinc supplements and 286 who took placebos. All of the women, ages 13 to 44, took a multivitamin-mineral tablet that contained no zinc.

The study found that babies born to the zinc group weighed an average of 4.5 ounces more than the other babies. Head circumference also averaged .16 inch greater.

Low birth weight is associated with infant deaths and such problems as cerebral palsy and mental retardation.

The biggest impact in the zinc study was among thin women, but researchers are not sure why.

The zinc supplements had little or no effect on infants of the heavier mothers studied. But women 5 feet 4 and less than 110 pounds, for example, had babies an average of 15 ounces heavier if they took zinc, the study found.

The infants of average-size women taking zinc benefited but less so. For example, women who were 5 feet 4 and less than 150 pounds had babies averaging almost 8.9 ounces heavier if they took zinc.

Goldenberg speculated thin women have less zinc stored in their bodies. Also, "The heavier the woman is, the more likely she is to have a large baby" anyway, he said.

Goldenberg said he would recommend that pregnant women who are especially thin take zinc supplements.

Exactly why zinc helps infants isn't known. But the researchers said a number of complex mechanisms are probably involved. Zinc, for example, plays a critical role in protein synthesis and immune functions, they said.