A new study from Columbia University's Mailman School of Public Health finds that more than one in five pregnant women in the U.S. smoke cigarettes, and 10 percent are dependent on nicotine.

Among pregnant women who smoked, 30 percent were diagnosed with mental illness, including personality disorders, major depression, and phobias. Among nicotine-dependent pregnant women, 57 percent were found to have mental disorders.

"Our research shows that prenatal smoking appears to be more common in pregnant women who are already vulnerable -- those who are unmarried, have less than high school education, and have lower personal incomes," added lead researcher Renee Goodwin. "They also are likely to have limited access to health care services, which may contribute to a lower likelihood of some women quitting smoking upon becoming pregnant."

The findings were published in the April 2007 issue of the journal Obstetrics and Gynecology.