Babies who survive often face serious and lifelong health problems.
According to the National Academy of Medicine, preterm birth accounts for more than $26 billion annually in avoidable medical and societal costs.
"This is a problem for our entire country," said Pellegrini. "For childless families, for people of all ages, for communities everywhere."
The national uptick led the U.S. to get a "C" rating on an "A" to "F" scale from the March of Dimes.
"We need to understand those issues more deeply so we can figure out how to support those women and ensure those babies aren't born too soon," said Pellegrini.
Across the country, preterm birth rates were nearly 48 percent higher among black women.
Rates were more than 15 percent higher among American Indian/Alaska Native women compared to white women.