By Amy Norton

NEW YORK (Reuters Health) - Smoking during pregnancy appears to affect children's birthweight, and possibly their risk of becoming overweight, but it may not directly harm other aspects of physical and cognitive development, a large study suggests.

The findings, from a study of nearly 53,000 U.S. children born in the 1960s, found that those whose mothers smoked during pregnancy were at higher risk of low birthweight -- a link that studies have long noted.

There was also evidence, albeit weaker, that these children were more likely than children of non-smoking women to be overweight by age 7.

Some past studies have found this link as well, though researchers can only speculate on the reasons; one theory is that nicotine may affect the fetal brain in a way that influences appetite control later in life.

On the other hand, the current study also found that mothers' smoking did not appear to directly affect other aspects of their children's development -- including intelligence, school performance and the risk of behavioral problems.

An initial look at the data did show associations between smoking during pregnancy and various developmental problems. But those links disappeared when the researchers factored in the family environment -- such as parents' income and education, and whether the child lived with both parents.

The findings suggest that other characteristics of families in which mothers smoke during pregnancy are what strongly influence children's long-term physical and cognitive development, rather than smoking per se.

None of this, however, diminishes the harm that smoking can cause, according to lead researcher Dr. Stephen E. Gilman, of the Harvard School of Public Health in Boston.

"Smoking still causes cancer; smoking still causes heart disease, And we still find a strong effect of smoking on birthweight, and we know that low birthweight can have negative consequences," Gilman told Reuters Health.

Low-birthweight infants are more likely to have medical problems shortly after birth, and some studies have linked low birthweight to a higher risk of certain health conditions later in life, such as high blood pressure and diabetes.

Gilman and his colleagues based their findings on data from a study that followed 52,919 U.S. children from birth to age 7. During pregnancy, their mothers reported on their smoking habits; and also provide information on family income, marital status, education, family history of mental illness and other factors that could affect their children's development.

A unique aspect of the study, Gilman noted, is that it included more than 2,000 sets of siblings whose mothers had smoked during one pregnancy but not the other. If maternal smoking, itself, affects children's IQ, school performance and other aspects of development, then differences should be apparent among these siblings.

The study found no evidence that this was the case, however.

Still, nothing about the findings changes the advice to women, according to Gilman: If they smoke, they should quit, for the sake of their own health and their babies'.

SOURCE: American Journal of Epidemiology, September 1, 2008.

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