The fertility rate in the U.S. fell to the lowest level on record last year, with women in their 20s having fewer babies, researchers at the Centers for Disease Control and Prevention announced Thursday.

Between 2022 and 2023, the fertility rate fell by 3%, a steeper drop than in previous years. In 2022, the rate held steady, and in 2021, the fertility rate increased by 1%, according to the CDC. Overall, U.S. fertility rates have been declining for decades, and the drop in 2023 followed historical trends, researchers told USA TODAY.

More women who are having babies are doing so in their 30s, the researchers found. Among women 20 to 24 there was a 4% decline in births.

For women in their 20s, "One of the big factors is the possibility of postponement," said Brady E. Hamilton, the lead author of the report.

Over the past few decades, and especially since the great recession of 2008, economic factors and societal expectations have led more people to conclude it's normal to have kids in your 30s, said Allison Gemmill a professor of family and reproductive health at Johns Hopkins University.

"It's OK to have children later, whereas maybe 20 years ago, 30 years ago, that just wasn't the norm," she said.

The fertility rate for 2023 is based on birth certificate data representing 99% of all births last year. Researchers said the data doesn't provide information on geography and it does not show whether a baby was a woman's first child.

Why did the birth rate fall?

We won't know precisely why fewer babies were born in 2023 than 2022 until social scientists gather data on parents' economic expectations and other factors that influenced why and when people had children, Hamilton said. That data will come out in the next few years, he said, from groups like the National Survey for Family Growth.

In 2023, there were 3.59 million babies born compared with about 3.67 million born in both 2022 and 2021, according to CDC data.

Record-low birth rate for women aged 20-24

Women aged 20 to 24 accounted for the greatest overall decline in the U.S. fertility rate: the birth rate among this age group, of around 55 births per 1,000 women, is the lowest on record, according to the CDC.

More women may be waiting to have children until their 30s and focusing more on school and career opportunities in their 20s, Hamilton said.

"The rates for older women tend to be more stable, and in general (are) increasing," he said.

It's also become harder for people in their 20s to find stable work in recent years, and it's taking some people longer to move out of their parents' home, Gemmill said. Polling and studies show people, including many members of Generation Z, do want to have children, they just want to have their professional lives in order first, she said.

"The transition to adulthood is a longer transition than it was 20 years ago," she said.

Women aged 30 to 34 had the highest birth rate in 2023, at 95 births per 1,000 women. In previous years, women aged 20 to 24 had the highest rate, Hamilton said.

Drop in births follows an uptick in 2021

Births fell in the first year of the COVID-19 pandemic, but in 2021, there was an unusual spike of 1%, most likely because people held off on giving birth the prior year, Hamilton said.

The 2021 uptick was a "rebound" following the uncertainty surrounding the COVID-19 virus and the economy in 2020, Gemmill said. She said economic factors heavily influence someone's decision to have a child.

The same people who accounted for 2020's decrease in births may have gone on to contribute to 2021's spike, she said.

"There was a time when, right when the pandemic started, people said, 'Oh, now's not a good time,' and then over time, they finally said, 'OK, maybe now,'" Gemmill said.

One-third of women had C-sections

For the fourth year in a row, more people delivered by cesarean section. C-sections accounted for 32.4% of births. In 2022, 32.1% of births involved a cesarean delivery.

From a health care perspective, it's not a good thing that more women are having C-sections, researchers said.

"Cesarean delivery is an invasive surgery and there are higher costs and more complications associated with it than with vaginal delivery," said Michelle Osterman, another author of the report.

The increase in C-sections could have happened because more women had babies in their 30s and 40s in 2023, and births in older women are more likely to require C-sections, Osterman said.

"Older women are having more births, so that might be playing into it," she said.