The preterm birth rate in the USA is especially high: 12.7% of all babies are born early, according to the March of Dimes. That rate has increased 36% in the past 25 years, partly because of an increase in elective cesarean section, an increase in older mothers and the growing use of assisted reproduction, which increases the risk of twins, triples and higher-order multiple births, the report says.

Most of the increase in the USA is in "late preterm" babies, born at 34 to 36 weeks of pregnancy, the report says.

Some babies are at higher risk than others, the report says. In the USA, black babies are 1½ times as likely as whites to be premature — a major reason that black infant mortality is so much higher than that of whites, says Christopher Howson, vice president for global programs at the March of Dimes.