As youth unemployment lags and student loan debt continues to grow, Millennials are delaying having children or choosing not to have them altogether.

Birth rates among American women in their 20s fell to the slowest pace in history as of 2012, according to a new report from the Urban Institute.

Between 2007 and 2012 the birth rate for women aged 20 to 29 declined by more than 15%. This marked an abrupt change from the prior three decades of fairly stable birth rates.

While birth rates among young women tend to drop during periods of economic stress, this past recession has seen a decline greater than those before. Yahoo Finance Senior Columnist Michael Santoli notes that the low birth rate among 20-something year olds could be a cyclical trend.

“We don't know…whether it was the impact of delayed adulthood and delayed incomes, and all the rest that came around the Great Recession and the overburden of student debt,” he says.

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It remains to be seen whether Millennials who put off childbearing during the recession could have more children in their thirties or if they will simply have fewer children than previous generations.

“The fact that people are delayed in getting started versus prior generations probably means fewer children altogether,” Santoli says. “But one reason it's not as big a concern is that Millennials themselves are a huge generation. So therefore there are still going to be a lot of people here being born, it's just not as many per mother.”

Santoli adds that the economic impact in terms of household formation could be great. “It's a multiplier effect in the economy, so we’ve been dealing with that lag,” he says.

He notes that it could take another 20 years to determine if we have “another baby bust generation,” characterized by slower growth rates due to less people driving business.