There's one more reason not to smoke during pregnancy.

A mother's cigarette smoking increases the risk that her newborn may have extra, webbed or missing fingers or toes, according to researchers at the Children's Hospital of Philadelphia

Just half a pack of cigarettes per day increases the risk to the baby by 29 percent, compared to non-smokers, although the overall risk of these abnormalities in fingers and toes is relatively low.

Because limbs develop very early in pregnancy, the effect may occur even before a woman knows she is pregnant, the researchers said.

"We found that the more a woman smoked, the higher the risk became that the baby would have these defects," said study leader Dr. Benjamin Chang, pediatric plastic and reconstructive surgeon at The Children's Hospital of Philadelphia. Dr. Chang and co-author Li-Xing Man, M.Sc., both of Children's Hospital and the University of Pennsylvania, reviewed the records of more than 6.8 million live births listed in the U.S. Natality database from 2001 and 2002. It is believed to be the largest study of its kind, covering 84 percent of U.S. births.

The researchers divided the study population into four groups: non-smokers, those who smoked one to ten cigarettes daily, 11 to 20 cigarettes daily, and 21 or more per day. There was a statistically significant dose-response effect, with increased odds of having a newborn with a congenital digital anomaly with increased maternal cigarette smoking during pregnancy.

Women who smoked up to half a pack a day were 29 percent more likely to have babies with digital anomalies and women who smoked more than a pack of cigarettes a day during pregnancy were 78 percent more likely to have babies with digital anomalies.

Of the total 6.8 million births, the researchers found 5,171 children born with digital anomalies whose mother smoked during pregnancy. "Overall, the likelihood of having a digital anomaly is relatively low, about one in 2,000 to 2,500 live births, and compared to other public health issues, is a very small problem," said Dr. Chang. "Usually surgery can restore full or nearly full function to children with these anomalies."

Limbs begin to develop between four and eight weeks of gestation and advance from a tiny nub to nearly fully formed fingers and toes. Many women only discover they are pregnant during this period.

Missing digits are twice as likely to occur in boys and are more common in Caucasians than African-Americans; more than five digits on hands and feet is 10 times more common in African-Americans and only slightly more common in boys.

The majority of isolated congenital digital anomalies occur spontaneously without any family history. The increased number of cases involving these diagnoses in their own practices led researchers to investigate environmental factors that might be associated with these conditions.

"Although the overall risk of having these defects is rather small, the increase in risk posed by tobacco exposure has the potential to affect thousands of children," he added.

"Health professionals should increase their efforts to remind women of the dangers of smoking."

The results of the study appeared in the January issue of "Plastic and Reconstructive Surgery," the journal of the American Society of Plastic Surgeons. The Children's Hospital of Philadelphia pediatric research program is among the largest in the country.

For more information visit www.chop.edu.