The age of newborns' dads in the country are on the rise.

FOX's Lilian Woo reports:

We've known that women have been delaying motherhood, now there's also more data out on dads. The average age of newborns' fathers was nearly 31-years-old in 2015, up by almost four years since the 1970's, according to a study published in the Journal Human Reproduction.

Researchers at Stanford University finding the number of newborns dads who are over 40 more than doubled over the past four decades, accounting for about nine percent of all U.S. births.

A study's co-authors says the trend existed for every race, ethnicity, education level and region of the country scientists looked at.

Doctor Michael Eisenberg also saying the findings could mean means fewer childbearing years and smaller families with potentially large economic and public health impact.

Lilian Woo, FOX News.

Follow Lilian Woo on Twitter: @LilianNY