Mother's smoking linked to child's IQ drop.

Preschool children whose mothers smoked heavily during pregnancy scored significantly lower on standardized IQ tests than kids whose mothers did not smoke, according to a new study.

This isn't the first time that researchers have suggested that a pregnant woman's smoking habits might have an impact on her offspring. Last year, a Canadian researcher reported that children born to women who smoked during pregnancy may have subtle auditory difficulties (SN: 7/10/93, p.23).

David L. Olds of the University of Colorado Health Science Center in Denver and his colleagues wanted to find out whether a maternal tobacco habit had an adverse effect on a child's intellectual ability later in life. From 1978 through 1980, the team enrolled 400 women pregnant with their first child.

The researchers asked each woman about her diet, smoking habits, and alcohol or drug use. To verify self-reported smoking behavior, the team measured a nicotine metabolite in urine samples collected from a subset of smokers enrolled in the study.

After delivery, the team continued to check on mothers and their offspring. When the children in the study reached preschool age, Olds and his co-workers began measuring intellectual ability with the Stanford-Binet IQ test.

They report in the February PEDIATRICS that IQ scores of 3- and 4-year-old children whose mothers smoked 10 or more cigarettes daily during pregnancy averaged 9 points lower than those of kids whose mothers did not smoke. When the team controlled for factors known to influence a child's test scores, such as maternal IQ and alcohol use, they found that they could explain some, but not all, of the difference. The children of mothers who had smoked still scored about 4 points lower than the offspring of nonsmoking mothers.

Four points doesn't seem like much, but it is equivalent to the deficit seen in children exposed to moderate amounts of lead, says coauthor Charles R. Henderson Jr. of Cornell University. That doesn't necessarily mean an individual child will have trouble in school, Henderson adds. At the same time, smoke-exposed children may not reach their.full intellectual potential, he says.

In a second study, which also appears in the February PEDIATRICS, the same research group tried to reduce the risk of childhood cognitive deficits by modifying the behavior of mothers who smoked.

Many of these women were visited frequently during and after their pregnancy by nurses, who counseled them about smoking and diet. The researchers discovered that such women cut back on their cigarette use and improved their diet. Moreover, their 3- and 4-year.old children had about the same average IQ scores as children whose mothers didn't smoke.

Reductions in maternal smoking explain part of the improvement in test scores; however, better nutrition may also play a role. "It would be nice to have a complete biological explanation, but we don't have that," Henderson says.

Further research must confirm the theory that mothers who smoke during pregnancy will impair their children's cognitive abilities. The reported link between maternal smoking and a child's performance on an IQ test years later may simply be a statistical fluke, the researchers note. The team plans to continue to look at IQ differences in the smokers' offspring, who are now in their teens.

On the other hand, something in tobacco smoke may harm the developing fetal brain. Cigarette smoke contains an estimated 2,000 to 4,000 chemicals, some of which could damage fragile fetal cells, Henderson notes. It may be that subtle smoke-induced damage doesn't show up until a child reaches age 3 or 4, when higher.order cognitive skills kick in, he adds.