If pregnant smokers need another reason to quit, a new study may have found it. The habit, researchers say, may raise the risk of having a baby with extra, missing or webbed fingers and toes.

Using information from a national database on U.S. births, researchers found that babies born to women who smoked during pregnancy were 31 percent more likely to have such birth anomalies as babies of non-smokers. And the more a woman smoked, the greater the risk.

Li-Xing Man and Dr. Benjamin Chang of the University of Pennsylvania in Philadelphia conducted the study, which is the largest one to date on smoking and birth defects of the fingers and toes. They report the findings in the medical journal Plastic and Reconstructive Surgery.

It’s estimated that about 1 in 600 infants are born with an extra finger or toe - an anomaly known as polydactyly. Webbed fingers or toes, called syndactyly, are less common, occurring in one in every 2,000 to 2,500 births. Adactyly refers to the absence of fingers or toes.

While it’s well known that prenatal smoking can have serious consequences such as miscarriage, premature delivery and low birth weight, studies have yielded conflicting results on whether smoking can cause birth defects of the fingers and toes.

Many of those studies, according to Man and Chang, have been limited by small study populations and shortcomings in methodology.

So for their study, the researchers analyzed information from a national database covering nearly all live births in the U.S. between 2001 and 2002. Of the more than 8.0 million births, maternal smoking information was available for 6.8 million.

A total of 6,522 infants had polydactyly, syndactyly or adactyly. Of these infants, 1,121 had other abnormalities, and were excluded from the analysis. Overall, 5,171 infants were included in the final analysis.

The risk, the researchers found, was not only greater among babies born to smokers, but it rose as the number of cigarettes a mother smoked during pregnancy increased.

Babies whose mothers smoked at least 21 cigarettes a day were most at risk, being 78 percent more likely than infants of non-smokers to have a finger or toe anomaly. But even relatively light smoking - 10 or fewer cigarettes a day - boosted the risk by 29 percent.

The extent to which smoking raised the odds of these birth anomalies was surprising, Chang said in a statement. “Our hope is this study will show expectant mothers another danger of lighting up,” he said.

SOURCE: Plastic and Reconstructive Surgery, January 2006.