Both smoking and obesity can harm unborn children, according to separate studies released Tuesday.
Pregnant women who smoke are 50 percent more likely to have mentally retarded children, according to one study.
Smoking during pregnancy previously was linked to low birth weight, infant mortality and lower intelligence in children. This study was the first to connect smoking with retardation, said Carolyn D. Drews, an associate professor of epidemiology at Emory University who headed the research.
The study - published in the April issue of Pediatrics, the journal of the American Academy of Pediatrics - found:
Women who smoked while pregnant were 50 percent more likely to have mentally retarded children.
About 35 percent of women who gave birth to retarded children reported smoking as few as five cigarettes a week during pregnancy.
Women who smoked during the last six months of pregnancy, when a fetus develops many organs, were 60 percent more likely to have retarded children than women who did not smoke in that period.
Pregnant women who smoked at least a pack of cigarettes a day were 85 percent more likely to give birth to a retarded child.
The researchers factored in elements believed to increase the chance of mental retardation, such as the child’s gender and race and the mother’s age, economic status, education and alcohol use.
The two obesity studies found that overweight mothers are at least twice as likely as thinner women to have babies with debilitating birth defects.
And a vitamin known to help prevent such abnormalities appears to offer overweight women no protection, the researchers said.
Both studies, published in today’s Journal of the American Medical Association, involved so-called neural tube defects and women who were obese at conception, not those who gained a lot of weight during pregnancy.
The neural tube defects linked to obesity include spina bifida - an incomplete closure of the spinal column that often results in paralysis - to anencephaly, in which most of the brain is missing. The defects annually afflict some 2,500 U.S. babies and an unknown number of miscarried fetuses.
An estimated 10 percent of women of childbearing age are obese.
“Considering the recent increase in obesity in the U.S. population,” Drs. Robert L. Goldenberg and Tsunenobu Tamura of the University of Alabama wrote in an accompanying editorial, the findings “may have important implications for the prevention of neural tube defects.”
Researchers who conducted the two studies offered no explanation for the apparent link between obesity and birth defects. But the Alabama doctors speculated that fat women might have some abnormality in their metabolism or that unrecognized nutritional factors might play a role.
Werler also found that at least 400 micrograms of folic acid a day - the recommended dose for women of childbearing age - reduced the risk of neural tube defects by 40 percent in women weighing less than 154 pounds. Folic acid did not reduce the risk in heavier women.
Folic acid, a trace B vitamin, is found in citrus fruits and dark, leafy vegetables.