The study authors said this positive trend occurred within one year of smoke-free policies being put into effect. They added that their findings show smoking bans have significant health benefits for both adults and children.
The study was published online March 28 in the journal The Lancet.
"Our research found significant reductions in preterm birth and severe asthma attacks in childhood, as well as a 5 percent decline in children being born very small for gestational age after the introduction of smoke-free laws," Dr. Jasper Been, of the Maastricht University Medical Centre, in the Netherlands, said in a journal news release.
"Together with the known health benefits in adults, our study provides clear evidence that smoking bans have considerable public-health benefits for perinatal and child health, and provides strong support for [World Health Organization] recommendations to create smoke-free public environments on a national level," Been said.
The researchers examined five North American studies on local smoking bans and six European studies focused on national smoke-free policies. The review involved a combined 2.5 million births and almost 250,000 asthma flare-ups among children.
Study co-author Professor Aziz Sheikh said the new study shows the potential that smoke-free laws have to decrease premature births and childhood asthma.
"The many countries that are yet to enforce smoke-free legislation should in the light of these findings reconsider their positions on this important health policy question," said Sheikh, of Brigham and Women's Hospital in Boston and the University of Edinburgh in Scotland.
The study authors said comprehensive smoking bans cover just 16 percent of the world's population, and 40 percent of children are regularly exposed to secondhand smoke. Young people account for more than 25 percent of all deaths and more than 50 percent of all healthy years of life lost to secondhand smoke exposure.
The U.S. Centers for Disease Control and Prevention provides more information on smoke-free policies.