- Americans aren't having enough babies to replace the population, according to a new report from the Centers for Disease Control and Prevention.
- The country's total fertility rate is 16% below what's required to keep the population stable.
- Experts have cited several reasons for the decline in births, including women delaying childbearing and better sex education resulting in fewer teen pregnancies .
- US birth rates have been below "replacement level" for decades, fueling fears that the country is headed for a " demographic time bomb ."
A new government report found that the US birth rate fell once again in 2017 , and that Americans still aren't having enough babies to replace the current population.
The report , published Thursday by the Centers for Disease Control and Prevention (CDC), focused on a measure called total fertility rate. That's the number of expected births a group of 1,000 women would have during their lifetimes, according to current age-specific birth rates.
In order to sustain a population, a country needs a total fertility rate of 2,100 births per 1,000 women, the report said.
But in 2017, the US rate was 1,765.5 per 1,000, or 16% below what's required to replace the population over time. (There were some wide swings between states, however: South Dakota had the country's highest total fertility rate at 2,227.5, and the District of Columbia had the country's lowest at 1,421.0)
Though birth rates in the US have been declining steadily for years, the 2017 overall rate represents both a seven-year low and the steepest decline in recent history , NBC News reported. In 2016, the US total fertility rate was 1,820.5 , and in 2015, it was 1,843.5 .
Experts suspect there are several forces behind the trend
In its report, the CDC did not say why the total fertility rate had declined, but health experts have offered some explanations for the trend.
In an interview with NBC News, Dr. John Rowe, a professor at Columbia Universitys Mailman School of Public Health, said shifts in women's societal role is one factor.
"In general women are getting married later in life, he told NBC News. "They are leaving the home and launching their families later."
Rowe added that the growth of sex education has resulted in fewer teen pregnancies.
"Weve been seeing, year after year, a precipitous drop in the number of births to teenage girls," he told NBC News. "That's good news."
Other experts have cited economic factors, like the 2008 recession and high education costs, as factors that may prompt Americans to delay childbearing. And in 2018 survey conducted by The New York Times, adults who want kids said they sometimes end up having few, or zero, kids due to the high cost of childcare.
Birth rates have long been below "replacement level," fueling fears of a "demographic time bomb."
Since the 1970s, birth rates in the US have been below "replacement level," or the rate at which new births keep the population steady.
This trend has sparked worries that the US may be headed for what's known as a "demographic time bomb," in which an aging population isn't replaced by enough young workers.
"An aging population will mean higher costs for the government, a shortage of pension and social security-type funds, a shortage of people to care for the very aged, slow economic growth, and a shortage of young workers," Harvard University sociologist Mary Brinton told Business Insider in 2017 .
The US is not the only nation facing such worries. Other countries including Spain, Bulgaria, and Latvia, may also be a risk of becoming demographic time bombs. South Korea, which has one of the world's lowest fertility rates, offers cash subsidies to entice families to have kids . Japan's birth rate has long been low, too, though it has inched upwards recently .
But one expert told NBC News the downward trend may change.
"I think it may stabilize once women who have been postponing pregnancy have the births they are planning to have," Donna Strobino, a professor at the Johns Hopkins Bloomberg School of Public Health.
As TIME's Jamie Ducharme and Fortune's Lucas Laursen both noted, the total fertility rate is based on current birth rates by age, so if women start having babies later and later in life, then the total fertility rate may underestimate future childbearing. And many US women are having babies later: In 2017, US birth rates increased only in women aged 40 to 49 .
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