Three-year-olds whose mothers smoked during pregnancy are physically and mentally less well developed than babies born to women who stopped smoking before their babies were born.

In a pair of reports published in the International Journal of Epidemiology, a team of researchers from the University of Maryland School of Medicine studied children whose mothers smoked continuously or quit smoking during pregnancy.

Because many of the mothers who quit smoking resumed after the baby was born, researchers were able to determine that some of the deficits stemmed from smoking during pregnancy. Toddlers whose mothers quit while pregnant fared better than those whose mothers never stopped at all.

The researchers measured the height and weight of 188 children of mothers who stopped smoking during pregancy and 526 whose mothers were "persistent smokers."

Cognitive tests were performed on 101 children whose mothers had stopped smoking and 263 whose mothers smoked right through the pregnancy.

In both groups, the heights, weights and cognitive scores were significantly higher when the mother had stopped smoking, even if she started again after the baby was born.

Mothers who smoked continuously also reported that their children had more language problems.

Principal researcher Mary Sexton, professor of epidemiology and preventive medicine at the University of Maryland medical school, said the studies "strengthened the possibility that a toxic effect from prenatal exposure {to tobacco smoke} results in both fetal growth retardation and damage to the central nervous system."

The cognitive differences, she said, were similar to findings on the toxic effects of lead exposure.

Previous studies have shown that babies born to mothers who smoke tend to be smaller than the newborns of nonsmokers.