Fifteen million babies are born prematurely each year, and the United States fared badly in the first country-by-country global comparison of premature births, which was released Wednesday by the World Health Organization and other agencies.

Although American hospitals excel at saving premature infants, the United States is similar to developing countries in the percentage of mothers who give birth before their children are due, the study’s chief author noted. It does worse than any Western European country and considerably worse than Japan or the Scandinavian countries.

That stems from the unique American combination of many pregnant teenagers and many women older than 35 who are giving birth, sometimes to twins or triplets implanted after in vitro fertilization, the authors said. Twins and triplets are often deliberately delivered early by Caesarean section to avoid the unpredictable risks of vaginally delivering multiple full-term babies.

Also, many American women of childbearing age have other risk factors for premature birth, like obesity, diabetes, high blood pressure or smoking habits. And the many women who lack health insurance often do not see doctors early in their pregnancies, when problems like high blood pressure or genital infections can be headed off.