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January 10, 2019, 05:53 GMT / Updated January 10, 2019, 1:06 GMT

By Linda Carroll and Shamard Charles, M.D.

Americans have fewer and fewer babies, according to a new government report. In fact, we do not have enough babies to replace us.

In order for the population to reproduce at current levels, the "total fertility rate" must be 2,100 births per 1,000 women of reproductive age, said researchers at the Centers for Disease Control and Prevention. report, released early Thursday. But the latest data indicate a current rate of only 1,765.5 per 1,000, or 16% less than the number needed to keep the population stable without an increase through immigration.

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

The CDC did not provide any explanation as to why the US fertility rate fell so precipitously.

Experts say that the decline is not due to a single cause, but rather to a combination of several factors, including changes in the economic situation, delays in the delivery of women seeking employment and their education, the greater availability of contraception and the decline of early pregnancies.

The trend in the United States is also found in many developed countries, including Western Europe, said Dr. John Rowe, a professor at Columbia University's Mailman School of Public Health. The changing roles of women in society, says Rowe, is a determining factor.

"In general, women get married later in life," he said. "They leave the house and start their families later."

But there is no guarantee that things will go as planned.

Dr. Helen Kim, an associate professor at the Feinberg School of Medicine at Northwestern University, said some of these women may eventually have two or more babies, but others may face a fertility problem.

"I think that as a delay in childbirth, women may not realize that fertility decreases with age and that there are limits to what the Fertility treatments can bring them, "Kim told NBC News on Wednesday. Although an increase in the birth rate among older women is reported, "this may not be enough to offset the decline" in births among younger women.

In addition, said Kim, the concept of the ideal size of the family could change. "There are changes where the trend is to have fewer families," she added. "I can not talk about it as a sociologist, but that's what I saw among my peers and colleagues."

The decline in early pregnancy is one of the most important factors, said Rowe. "We are seeing, year after year, a sharp drop in the number of teenage births," he added. "This is good news – not only do these kids have none, but they also have the chance to finish high school, and that makes a huge difference in their lives."

Rowe attributes the significant growth of sex education in schools to the sharp drop in the number of early pregnancies.

Some suspect that the downward trend may falter or even reverse. "It may not be totally catastrophic," said Donna Strobino, a professor at Johns Hopkins Bloomberg School of Public Health. "I think it could stabilize once the women who have postponed their pregnancies have the births that they plan to have."

And if women's fertility declines with age, younger women have an option that was only available recently: freezing eggs.

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

The new report is not limited to the entire nation. It also broke down total fertility rates by state, ranging from a high of 2,227 in South Dakota to a low of 1,421 in Washington, D.C.

Part of this variation could be related to the passage of the Affordable Care Act, Strobino said. "Some states have not chosen to extend Medicaid," she explained. "And that would have had an impact on access to family planning services."

Another possible explanation for this variation might be that more educated women tend to live in particular states, Strobino said. Washington, for example, tends to attract more educated women "because of the labor market," she said.

CORRECTION (January 10, 2019, 9:50 am ET): An earlier version of this article inaccurately indicated the number of births required to maintain a population at current levels. That's 2,100 births per 1,000 women of reproductive age in their lifetime, and not per year.