Americans are having fewer and fewer babies, a new government report finds. In fact, we now aren’t making enough babies to replace ourselves.

For the population to reproduce itself at current numbers, the “total fertility rate” needs to be 2,100 births per 1,000 women of childbearing age over their lifetime, researchers for the Centers for Disease Control and Prevention said in their report, released early Thursday. But the latest data show a current rate of just 1,765.5 per 1,000, or 16 percent below the number needed to keep the population stable without additions through immigration.

The total fertility rate has been declining steadily for seven years, but the numbers for 2017 represent the biggest drop in recent history. The rate for 2016 was 1,820.5; for 2015, 1,843.5; and for 2014, 1,862.5.

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The CDC offered no explanation for why the American fertility rate is dropping so precipitously.

Experts say the decline isn’t due to a single cause, but rather a combination of several factors, including changing economics, delays in childbirth by women pursuing jobs and education, the greater availability of contraception, and a decline in teen pregnancies.

The trend seen in the United States is also seen in much of the developed world, including Western Europe, said Dr. John Rowe, a professor at Columbia University’s Mailman School of Public Health. One important factor driving this is the changing roles of women in society, Rowe said.

“In general women are getting married later in life,” he explained. “They are leaving the home and launching their families later.”

But there’s no guarantee that things will work out as planned.

While some of these women may eventually have two or more babies, others may hit a fertility wall, said Dr. Helen Kim, an associate professor at Northwestern University’s Feinberg School of Medicine.

“I think as women delay childbearing they may not realize that fertility declines with age and that there are limits to what fertility treatments can do for them,” Kim told NBC News on Wednesday. Although there are reports of increased birth rates among older women, “that may not be enough to make up for the decline" in births among younger women.

Moreover, Kim said, the concept of the ideal family size may be changing. “There are shifts where having smaller families is a trend,” she added. “I can’t speak on this as a sociologist, but this is what I’ve seen among my peers and colleagues.”

One of the biggest factors is the decline in teen pregnancies, Rowe said. “We’ve been seeing, year after year, a precipitous drop in the number of births to teenage girls,” he added. “That’s good news. Not only are these children not having children, but they’re also getting a chance to finish high school. And that makes a huge difference to their lives.”

Rowe credits the growth of sex education in the schools for that big drop in teen pregnancies.

Some suspect that the downward trend may peter out or even reverse itself. “It may not be all doom and gloom,” said Donna Strobino, a professor at the Johns Hopkins Bloomberg School of Public Health. “I think it may stabilize once women who have been postponing pregnancy have the births they are planning to have."

And while women’s fertility takes a hit with age, younger women have an option that wasn’t available until fairly recently: egg freezing.

In vitro fertilization "is becoming more and more popular,” said Dr. James Grifo, program director for the NYU Langone Fertility Center. And “more young women are choosing to freeze their eggs to protect themselves in the future.”

The new report didn’t just look at the nation as a whole. It also broke down total fertility rates by state, which ranged from a high of 2,227 in South Dakota to a low of 1,421 in Washington, D.C.

Part of that variation may be related to the passage of the Affordable Care Act, Strobino said. “Some states did not select to expand Medicaid,” she explained. “And that would have impacted access to family planning services.”

Another possible explanation for the variation may be that women with more education tend to live in particular states, Strobino said. Washington, for example, tends to attract more educated women “because of the job market,” she said.

CORRECTION (Jan. 10, 2019, 9:50 a.m. ET): An earlier version of this article misstated the number of births needed to maintain a population at current levels. It is 2,100 births per 1,000 women of childbearing age over their lifetime, not per year.