Birth rates in the U.S. fell for the fourth year in a row in 2018, according to data released by the Centers for Disease Control and Prevention (CDC) on Wednesday.

The total number of babies born in the country is the lowest in more than three decades, according to the CDC’s National Center for Health Statistics. There were 3,791,712 babies born in the U.S. in 2018, down 2 percent from 2017. That number marks the lowest number of births in 32 years, according to the CDC.

The total fertility rate in 2018 declined to 1,729.5 births per 1,000 women. The fertility rate refers to how many children women have overall, while the birth rate says how many women have in a one year, according to NBC News.

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The data show that Americans are not having enough children for the population to remain stable. According to the CDC, there would need to be 2,100 births per 1,000 women in order for the population to not decline.

The drop is driven by women waiting longer to have children, into their 20s and early 30s, as well as a decline in the teen birth rate. Births dropped approximately 7.4 percent for girls and women between the ages of 15 and 19 between 2017 and 2018.

The number of births increased slightly in 2018 for women 35 to 44. Alison Gemmill, a demographer at Johns Hopkins University, said the data shows that 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,” Gemmill told The New York Times.

When broken down by race, fertility rates declined 2 percent for black and white women, 3 percent for Hispanic women and 4 percent for Asian women between 2017 and 2018.

The data released by the CDC does not specify whether all of the people who gave birth in 2018 identify as women.