The U.S. fertility rate just hit a historic low. Why some demographers are freaking out.

The debate now is about whether the United States is headed toward a “national emergency,” as some have feared, or whether this is a blip and the birthrate will level off soon.

“It’s about millennials,” says Donna M. Strobino, a professor of population, family and reproductive health at the Johns Hopkins Bloomberg School of Public Health.

Those supposedly entitled young adults with fragile egos born in the 2000s who live in their parents’ basements and hop from job-to-job — it turns out they’re also much less likely to have babies, at least so far. Some experts think millennials are just postponing parenthood while others fear they’re choosing not to have children at all.