The characters of the TV show Girls have the kinds of problems only childless people complain about. A fight with a partner over whether the songs you write together sound like those of a popular indie band. A fight with a friend over how long it took them to come over. A chilly reception to one's modern-art show.

It's no wonder Millennial audiences relate to the show so well: Today's twentysomething women have been slower to have children than any previous generation.

In a new report, the Urban Institute think tank writes that in 2012, there were only 948 births per 1,000 women in their 20s, "by far the slowest pace of any generation of young women in U.S. history." In 2007, the rate was 1,118 births per 1,000. The decline in births was largest among Hispanic women, at 26 percent, followed by black women, at 14 percent, and an 11 percent drop for white women.

Decline in Fertility Among Twentysomething Women, by Race

Urban Institute

The researchers put forward a few theories for the decrease. One is the Great Recession, which might have hit pause on pregnancy plans because babies are, among other things, expensive. Past studies have estimated that the recession led to a 2.4 percent decrease in the fertility rate, or about 426,850 live births. The recession also slowed the trickle of immigrants to the U.S., and immigrants tend to have more children.