Every year, roughly 15 million babies are born prematurely.
And now, scientists think the air we breathe is partially to blame.
Estimates in a new study have linked up to 3.4 million premature births to pregnant women breathing fine particulate matter — or air pollution.
According to the study, the highest numbers of those births are in South and East Asia, the Middle East and some African countries.
The study does have limitations. There's not a whole lot of research on preterm births in some highly affected areas. Other factors like poverty, infection, education and psychological health also come into play.
Still, researchers believe a reduction in air pollution in the areas where it's most prevalent could mean fewer babies being born too early.
And that's an important finding. According to the World Health Organization, complications from premature births are the leading cause of death for children under 5.