DENVER (KDVR) — Colorado is becoming less and less of a place for the youngest age cohort.

Colorado’s fertility rate plummeted by 23% from 2010 to 2020. Only Arizona had a bigger drop in fertility.

Surrounding states have seen the same trend develop, while some U.S. regions have seen fertility drop much faster than others. Western states – those that touch the Rocky Mountains and farther west – have seen fertility rates decline by almost twice as much as the rest of the U.S.

The U.S. Census Bureau released new figures on Thursday detailing the nation’s population changes by age, sex and race. Between 2010 and 2020, the U.S. population that was 5 years old or younger dove by 8.9%. Nationwide, 5.6% of the U.S. population was 5 or under in 2020.

In Colorado, that population declined by 8.5% in the same time. In 2020, the 5 and under population in Colorado was 5.4% of the total – tied for 11th lowest in the nation with California, the District of Columbia, Michigan, New Jersey, New Mexico and South Carolina.

Midwestern and southern states had the highest rates of children under 5 outside of Utah, which has the highest share at 7.3%. New England and Florida have the lowest rates.

Among Colorado’s counties, mountain counties have the lowest shares of 5 and under residents, while plains and northern Colorado counties have the highest.

San Juan, Crowley, Ouray, Gilpin and Huerfano counties have the lowest, while Kiowa, Morgan, Weld, Yuma and Prowers counties have the highest.