Why does America have so many more premature births than Europe?

"Premature births are the chief reason the U.S. ranks 30th in the world in infant mortality, with a rate more than twice as high as infant mortality rates in Sweden, Japan, Finland, Norway and the Czech Republic."

The AP lists possible reasons suggested by "experts":

-Fertility treatments and other forms of assisted reproduction probably play a role because they often lead to twins, triplets or other multiple births. Those children tend to be delivered early.

-The U.S. health care system doesn't guarantees [sic] prenatal care to pregnant women, particularly the uninsured, said Dr. Alan R. Fleischman, medical director for the March of Dimes.

-Maternal obesity and smoking have been linked to premature births and may also be a factor.

-Health officials are also concerned that doctors increasingly are inducing labor or performing C-sections before the 37th week. However, Fleischman said most infant deaths do not occur in babies just shy of 37 weeks gestation, but rather in those much younger....

While "smoking" makes sense as one of the factors causing premature births in the US, it doesn't make sense as an explanation of why America has more premature births than Europe. Europeans smoke much more than Americans, and I'd assume there's a direct correlation between a country's overall smoking rate and the prevalence of pregnant women smoking. There must be factors aside from smoking that are so harmful to babies that they go even further than canceling out America's advantage in having relatively few smokers.