GEOFF BENNETT: The U.S.

fertility rate has dropped to an all-time low, according to new CDC data.

Researchers say it's not a blip.

It's a generation of women choosing to wait or not have children at all.

Our William Brangham has more.

WILLIAM BRANGHAM: That's right, Geoff.

This trend has been going on for a while.

Since 2007, the number of Americans having babies has dropped 23 percent.

And that means not enough kids are being born to replace older workers or those that die.

If this trend continues, or unless immigration levels change, the nation will have eight million fewer residents by 2055.

But there are some nuances to these numbers.

There was a 7 percent drop in teen pregnancies last year, which many experts say is a very good thing.

So, for more on what this means for current and future generations, we are joined by Brian Mann of NPR.

He's been reporting a long investigative series about global population trends.

Brian, welcome back to the show.

The U.S.

population has been on this significant downward trend for almost two decades.

You have spent a good deal of time looking into this.

What does your reporting say about what is driving this?

BRIAN MANN, NPR: Yes, there's a mix of factors, William, that are causing women and young couples to choose to have fewer children, smaller families, and, in some cases, no children at all.

And, as you say, this is already affecting the fabric of the country.

The population of the United States is aging rapidly.

The growth of the population has slowed dramatically already.

And, as you say, later this century, the population of the United States will actually begin to shrink if we stay on this path.

And that change, that shrinkage is going to come much sooner now than once predicted because there are fewer and fewer babies being born and also we have begun to squeeze immigration.

WILLIAM BRANGHAM: I mean, what are the implications here?

I mean, I guess I'm asking, how serious is this?

When does this become not just a demographic story, but something more serious?

BRIAN MANN: I think many of the economists I'm talking to think this is a right now thing.

This is not something where we're talking about 10 years or 20 years from now.

I mean, that statistic that we cited at the top of the segment, 710,000 fewer babies every year, those are fewer children going into public schools.

Those are eventually fewer workers who are going to be turning up in the economy.

And as the population of Americans age 60 and older grows rapidly, we're actually already beginning to see the population of Americans under the age of 25 begin to shrink.

And so we have really lived in a youth culture.

The United States since the baby boom after World War II, we have shaped our culture, our innovation, our industries by young people getting out, taking risks, starting new businesses, building the economy.

We're really pivoting now faster and faster toward a geriatric culture, where old people like myself, we're the growing population.

And that is going to mean a United States in the years ahead that many people say will be unrecognizable.

WILLIAM BRANGHAM: Does your reporting detail the rationales why people seem to say and tell demographers why they're not having children?

Is this an economics issue?

What is it causing people to not have so many kids?

BRIAN MANN: Yes, it's been interesting to sit down in living rooms with young women, with couples who are talking through these decisions.

And a lot of it is economics.

Affordability is something that comes up again and again.

Housing costs are soaring.

It's hard to get that starter home, where you could envision having a couple of babies and getting your family started.

Childcare is more expensive every year.

There are also big cultural shifts.

It used to be that there was just a lot of social pressure for women in their 20s to have more children, to have children sooner.

Now what we're seeing -- and this is a remarkable shift -- we now see half of all American women reaching age 30 without being moms.

Some of those women are having babies later.

We're seeing actually some increases in motherhood in age 35 and older, but not enough to offset this really stunning drop in women in their 20s who are opting out.

It's important to say this is not all downside.

A lot of those women, because they don't have children or they have fewer children, are getting more education.

They're starting careers.

They're building businesses and lives.

And so there are trade-offs here.

It's demographically very challenging for the country, but for some women and some couples, there's a big payoff here.

WILLIAM BRANGHAM: And, to that same point, the declining teen birth rate is good news, right?

BRIAN MANN: I think it's astoundingly good news.

This is a public health victory, a social win that in the United States we have been looking for, for decades.

Long after a lot of other developed countries had solved the teen pregnancy problem, the United States was still seeing a lot of very young women and even children having babies.

This one-year decline of 7 percent, that builds on improvements that have been happening for years.

So, yes, it affects the demographics of the country, but I think this is one trade-off that everyone is happy with.

And, again, what some demographers hope is that some of those young women will then choose to have babies later, when they are more educated, they have more successful economic lives.

There will be a more appropriate moment for motherhood, not when they're still in high school, not before they have really started their adult lives.

WILLIAM BRANGHAM: That is NPR's Brian Mann.

Brian, thanks, as always, for sharing your reporting with us.

BRIAN MANN: Thanks for having me.