Of the nearly 1,800 women in the study, those who quit smoking did gain about six more pounds than those who continued to smoke, and they gained roughly the same amount in the year after delivery. But their babies weighed the same, on average, as the babies of moms who never smoked. Low-birth-weight babies face higher risk for infections, respiratory disorders, and learning disabilities.
"The big thing to get out of this study is that quitting early in pregnancy is as helpful in respect to the birth weight of your baby as never having smoked while you were pregnant," Dr. Amber Samuel, a maternal-fetal medicine expert at Emory University School of Medicine in Atlanta, said.
"I think that can be an inspiration to moms who are looking to make a change in their lives."
According to the American Cancer Society, between 10 and 15 percent of women smoke during pregnancy. Studies have linked smoking to premature birth and other complications, such as birth defects, low birth weight, and stillbirth.
Infants have a three to four times higher risk of dying from sudden infant death syndrome, or "crib death," if their mothers smoke during and after pregnancy. Children exposed to secondhand smoke also have more ear infections, pneumonia, bronchitis, asthma, and other health problems.
The new study included 1,774 women who were part of the "Smoke-free Newborn" study conducted in Copenhagen, Denmark, between 1996 and 1999.
Twice during pregnancy, researchers surveyed women about their smoking status. To double-check whether women who said they quit smoking really did, their saliva was checked for cotinine - created when nicotine is broken down in the body.
About 38 percent of women were smokers before becoming pregnant, and half of them quit right before or soon after, Dr. Line Rode of Copenhagen University Hospital and colleagues found.
During pregnancy, nonsmokers gained almost 30 pounds, on average, smokers gained 29 pounds and quitters gained 35 pounds.
Image: Pregnant belly with cigarette, via Shutterstock