Pregnant women who smoke half a pack of cigarettes a day or more increase the risk of delivering a baby with deformed, extra or missing fingers or toes, HealthDay News reported Aug. 17.

Researchers said babies born to smokers are 30 percent more likely to have finger or toe deformities. "One of the things that smoking does is interfere with oxygen delivery to cells at very key moments in development," said researcher Manuel Alvarez of Hackensack University Medical Center in New Jersey. "If cells are deprived of oxygen, they don't proliferate as they should. If cells don't proliferate, you can have limb deformities."

Finger and toe deformities are among the most common birth defects: about 1 in 600 babies are born with an extra finger or toe, for example. Such defects often occur in families with no past history of such birth defects, so experts think that environmental factors may be the cause.

The study, which was based on a review of about 7 million birth records by University of Pennsylvania researchers, appears in the journal Plastic and Reconstructive Surgery.