Foreign-Born Mothers Raising US Birthrate, Study Finds

— -- Foreign-born mothers living in the U.S. are driving long-term population growth, according to a new report by Pew.

While overall the number of babies born in the U.S. increased from 3.74 million in 1970 to 4 million in 2014, the uptick is largely due to the number of births among foreign-born women in the U.S., which grew more than threefold. Births among U.S.-born mothers decreased.

Pew found that in 2014, "23 percent of all babies born in the U.S. had immigrant mothers, up from just 7 percent in 1970."

In 1970 there were 3.46 million births to U.S.-born women and a population of about 203 million, versus 3.1 million in 2014 and a total population of about 323 million.

"Almost one-quarter of births are to women who were not born in this country," said Gretchen Livingston, a senior researcher at Pew Research Center and the lead author of the report.

"In the long term, overall births annually in the U.S. would have been on the decline," she told ABC News. "Had there not been immigrant moms coming in, births would have actually dropped."

The researchers also found that of the 23 percent of babies born to immigrant women, roughly one-third were to undocumented immigrant women and the other two-thirds were in the U.S. with legal status. That works out to 7 percent of all U.S. births from undocumented immigrant women — down from 2007, when that figure peaked at 9 percent of U.S. births, or 370,000 babies.

The vast majority of foreign-born new mothers have been in the U.S. long term, more than 11 years, according to the study.

The researchers didn’t break down how long the undocumented immigrant mothers had been in the U.S. but point to a September report that found the vast majority of undocumented residents have been in the U.S. for more than 10 years.

"We are not seeing evidence of women crossing the border to have babies. This is not who the unauthorized immigrant population is," Jeffrey S. Passel, a senior demographer at Pew Research Center, told ABC News. "It's people who have been here a long time."

The researchers also found that the rate for births outside marriage declined for immigrant women — largely because of the changing geographic makeup of immigrants in the U.S.

A rising share of foreign-born mothers originated from Asian countries, where births outside marriage are quite low, compared with births from Latin American mothers.