Many women immediately quit smoking when they find out they are pregnant, knowing the habit is unhealthy for their unborn child. But sometimes that concern takes them only so far.

A new study has found that women who were unmotivated to remain smoke-free after the birth of a child were more concerned about their weight than those who intended to kick the habit for good.

"Thinking about weight is important in understanding women's smoking after pregnancy," said Michele Levine, a professor of psychiatry and psychology at the University of Pittsburgh Medical Center and lead author of the study. "Women are complex creatures. More than just whether they are going to breastfeed the baby, or are addicted to nicotine, or are addicted to alcohol, healthcare providers need to think about psychological issues like weight worries."

Levine said a normal woman would have to gain 80 pounds to 100 pounds to equal the risk of smoking postpartum.

For the study, which will appear in the October issue of the Annals of Behavioral Medicine, Levine and colleagues interviewed 119 women who had smoked at least eight cigarettes a day but who quit when they found out they were pregnant. During the women's third trimester, researchers asked whether they planned to resume smoking after the birth.

They found that 65 percent of the women were highly motivated not to smoke again, with 74 percent of that group confident they would be able to stick to their plan. The 35 percent who were less motivated to stay smoke-free after giving birth were more concerned about managing weight than those who were determined to give up cigarettes for good.

"The big question is what they actually do," Levine said. "But at the third-trimester point, weight concerns were more related to motivation to quit smoking than any other factor."

Researchers found that the more confident a woman was about her ability to maintain her weight without smoking, the more likely it was that she planned to quit for good. Researchers also found that women who intended to breastfeed were more likely to plan to abstain from smoking.

Dr. Sharon Phelan, an obstetrics-gynecology professor at the University of New Mexico and a member of the American College of Obstetrics and Gynecology, said a percentage of women who smoke do so to control their weight. When they get pregnant, she said, they get the support they need to quit because many people are aware that smoking carries health risks for a fetus. (The practice can decrease the amount of oxygen that gets to the baby, which interferes with fetal growth and can lead to pre-term delivery.)

"The irony is, after they have the baby, there is not that external support," Phelan said. "Friends don't understand why they won't join in for a cigarette, and the father of the baby, who was supportive during the pregnancy, no longer is. That makes it hard for a woman to stay smoke-free."

Dr. Jeanne Ballard, who consults for a quit-smoking hot line in Indiana that counsels pregnant women, said women need to be reminded there are other ways to lose weight: Breastfeeding, for example, uses 500 calories to 600 calories a day.

Post-birth, exposure to secondhand smoke can increase a child's susceptibility to chest colds and ear infections, experts say. It can also increase their risk of asthma and crib death.