Researchers from The Miriam Hospital's Centers for Behavioral and Preventive Medicine in Providence, R.I., said that 11 percent to 30 percent of women continue to smoke during pregnancy, the Centers for Disease Control and Prevention.

"A baby who is harder to soothe and more irritable could be more difficult to take care of and could potentially affect the developing mother-child relationship, especially for mothers who are already stressed and have fewer resources," lead author Laura Stroud said in a statement.

"We need better treatment programs to help women not smoke during pregnancy, to keep them from starting smoking after the baby is born, and to help them take care of an excitable or colicky baby."

In the study, Stroud and colleagues from Women & Infants Hospital in Providence and the Warren Alpert Medical School of Brown University focused on newborns between 10 and 27 days old.

All 56 babies -- 28 smoking-exposed and 28 unexposed -- were healthy and full-term. Maternal social class, age and alcohol use were similar in each group.

Postnatal smoke exposure was quantified by infant saliva cotinine levels -- the primary metabolite of nicotine.

The study, published in the Journal of Pediatrics, found that smoking-exposed infants showed a greater need for handling, or external intervention, in order to be soothed and calmed down.