A recent study in the Archives of Pediatrics and Adolescent Medicine (2007; 161 [1], 58–64) found that prenatal vitamin supplementation may help underweight pregnant women increase the birth size of their offspring and help prevent early neonatal mortality.

After studying 200 underweight pregnant women with a body mass
index of less than 18.5, the researchers concluded that the infants born to women who took vitamin and mineral supplements were heavier by 98 grams (g), or about 3.5 ounces, than those born to women who took a placebo.
The incidence of early neonatal morbidity also declined—from 28% to 14.8%—in the supplement group.