For the first time, women in their early 30s are having more babies than younger moms in the United States.

Health experts say the shift is due to more women waiting longer to have children and the ongoing drop in the teen birth rate.

For more than three decades, women in their late 20s had the highest birth rates, but that changed last year, according to preliminary data released Wednesday by the Centers for Disease Control and Prevention.

The birth rate for women ages 30 to 34 was about 103 per 100,000; the rate for women ages 25 to 29 was 102 per 100,000. The CDC did not release the actual numbers of deliveries for each age group.

Earlier this year, Statistics Canada estimated the average fertility rate based on 2016 census figures  at 1.6 children per woman, up slightly from the record low  of 1.51 in 2001. 

The fertility rate refers to the number of children a hypothetical woman would have over the course of her reproductive life, based on females aged 15 to 49.

In the 1960s, the average age for a first birth in Canada was about 22. By 2011, it edged up to age 30.2.

A separate CDC report focusing on deaths found the overall death rate in the U.S. fell last year after an unusual and worrisome increase in 2015.

The reports are based on a first look at birth and death certificates filed across the U.S. last year.