The rate of preterm births is rising in the U.S. mostly because of Cesarean section births, according to an analysis of nine years of data.

Researchers from the March of Dimes and the Centers for Disease Control and Prevention said that from 1996 to 2004 there was an increase of nearly 60,000 preterm births per year. Ninety-two percent of those were C-sections.

During that time, preterm births rose 10 percent, while C-sections rose 36 percent.

More than 520,000 babies -- one out of every eight -- are born too soon each year in the U.S., a news release said.

Late preterm babies, those born 34 to 36 weeks gestation, account for most of the increase in the preterm birth rate. These infants have a greater risk of breathing problems, feeding difficulties, temperature instability, jaundice, delayed brain development and death than babies born at term.

C-sections are the most common major surgical procedure for women. More than 30 percent of the 4.1 million U.S. live births are delivered via C-section, and the rate has increased dramatically since 1996.