The US birth rate has fallen for the fourth year in a row – the lowest point in 30 years. Not enough babies are being born in the US to replace the existing population.

A Centers for Disease Control and Prevention report released on Wednesday revealed that the fertility rate dropped to a record low in 2018, with 2% fewer births.

The total fertility rate, the number of children a woman will have in her lifetime, had also dropped, the CDC report revealed. For 2018, that figure stood at 1,729.5 births per 1,000 women aged 15 to 44, which was the fourth year the rate had decreased. To keep the current population stable, there would need to be 2,100 births per 1,000 women.

Alison Gemmill, a demographer at Johns Hopkins University, told The New York Times the data shows Americans who give birth “want to establish themselves before having children.”

“They also want to make sure they have adequate resources to raise quality children,” she added.

The median age of first marriages has climbed from 21 to 28 for women since 1970 and 23 to 30 for men, likely one reason in the decline.