AMNA NAWAZ: The fertility rate in the U.S. reached an all-time low last year, according to new CDC data released this month.

That trend has sparked fear among many on the right, from Elon Musk to Donald Trump.

They believe Americans should have more babies.

Others are concerned about the potential consequences, especially for the U.S. for women.

So is the declining birth rate a problem and how?

For the final part of our series The Next Frontier, special correspondent Sarah Varney looks at this movement encouraging families to have more children, its growing influence and its ties to the anti-abortion rights movement.

SIMONE COLLINS, Pronatalist: Yes, on this side, we have our blue-eggers.

SARAH VARNEY: After a visit to the chicken coop.

Wow.

That's amazing.

And a stroll through the field behind Simone and Malcolm Collins' late 18th century home in suburban Pennsylvania, talk quickly turns to their vision for the future.

SIMONE COLLINS: I'm mother to our four, soon five kids.

SARAH VARNEY: They want others to follow their lead.

SIMONE COLLINS: People need to fundamentally rethink what it means to live a successful life, for example, starting with family, starting with marriage, instead of starting with university, college debt, career, try to get out of debt, maybe eventually get married and then five years later try to have children while realizing your fertility window is already gone.

It's not ideal.

SARAH VARNEY: The Collinses are a part of a movement known as pronatalism that encourages people to have more children.

MALCOLM COLLINS, Pronatalist: We're pointing to a concerning trend line that if it continues to go in this direction is going to break a lot of the systems that our civilization has come to depend on and hundreds of millions of people are going to die as a result.

SARAH VARNEY: The birth rate in the United States has been declining since the late 1950s.

And, today, American women have on average 1.6 children.

That's below what's needed to replace the current population.

Nicholas Mark is a demographer at the University of Wisconsin-Madison.

NICHOLAS MARK, University of Wisconsin-Madison: Yes, there's no doubt that if we had to pick like increasing or declining, we would pick declining.

That is absolutely true.

But people should not be concerned about the U.S. population into the future.

SARAH VARNEY: Several trends are driving down the birth rate.

Women have increased access to contraceptives like IUDs, far fewer teenage girls are having babies, and more women are having children in their late 20s and 30s.

NICHOLAS MARK: What we don't know is whether those people who have fewer kids at younger ages are going to go on and have more kids at later ages.

And that we simply cannot answer because we don't know what they're going to do when they get older.

SARAH VARNEY: Even still, this panic among conservatives has been stoked by former Trump adviser and tech billionaire Elon Musk.

ELON MUSK, Owner, X: The birth rate is very low in almost every country.

And so unless that changes, civilization will disappear.

SARAH VARNEY: And now the White House is readying its own plans.

DONALD TRUMP, President of the United States: And we will support baby bonuses for a new baby boom.

How does that sound?

That sounds pretty good.

I want a baby boom.

(CHEERING) ROGER SEVERINO, Vice President of Domestic Policy, The Heritage Foundation: Should we care if we're not going to be replacing ourselves?

The answer is resoundingly absolutely, yes.

SARAH VARNEY: Roger Severino served in the first Trump administration.

He and his Heritage Foundation colleagues are urging the White House to adopt what they call family-friendly policies.

ROGER SEVERINO: So you have now a reverse pyramid, where the numbers of children that will be workers is smaller than the number of people retiring.

So what does that do to Medicaid, Medicare, Social Security, all of these programs that are premised on the notion that we have a large, growing population of workers to contribute to that system?

We're going to have young and old be the next frontier of fights over resources.

NICHOLAS MARK: Even if we had more babies right now, those babies are not going to be paying taxes for at least 20 years-ish.

SARAH VARNEY: But Nicholas Mark says there is a quicker, easier fix.

NICHOLAS MARK: If you think that you have a problem with paying for entitlements right now, there are millions of people around the world who want to move to this country and want to pay taxes to the U.S. government right now.

And that is an immediate answer to a question that changing fertility rates does not answer at all in the short term.

SARAH VARNEY: Bethany Mandel and her husband, Seth, both Orthodox Jews, live in suburban Maryland with their six children.

On top of homeschooling her kids, Mandel is a podcast host who wants to show people how rewarding having a big family can be.

BETHANY MANDEL, Host, "Mom Wars": People say all the time, like, oh, your family is like really chaotic, but also really loving and really fun.

SARAH VARNEY: Studies show that many factors are keeping Americans from growing their families, like stagnant wages, rising rents and home prices, and financial insecurity.

But Mandel says there are steps the government could take to ease the burden, like increasing the child tax credit.

BETHANY MANDEL: Under the first Trump administration, it was much better.

And then those tax credits expired and were right back to where we were.

It's $2,000.

And if you think that it costs $2,000 to raise a child in America, I would like you to come sit at my kitchen table and talk the budget with me.

SARAH VARNEY: Sure.

Other countries, including South Korea, Japan, Poland and Sweden, have tried all sorts of measures to increase fertility rates, but with little success, including subsidized childcare, paid parental leave and cash bonuses.

LESLIE ROOT, University of Colorado Boulder: But there's this idea that it's coming in from the West and it has to be rooted out.

SARAH VARNEY: Leslie Root is a demographer at the University of Colorado Boulder.

LESLIE ROOT: It's just not enough to incentivize somebody to have a birth that they weren't going to have otherwise.

So you end up with this bump in birth rates, but it sort of goes away because you aren't actually catching very many people at the margins who were on the fence about having another kid.

SARAH VARNEY: Nicholas Mark says fertility rates aren't a lever that can be pulled.

NICHOLAS MARK: That's not how these monumental and personal decisions work, right?

These are decisions that are made in the context of people's entire lives, in the context of the whole society that they live in.

It's not like an interest rate that you can ping up and down, which then will trickle down into having effects on unemployment.

That is -- it's not how it works.

SARAH VARNEY: A big part of the solution, says Roger Severino,is marriage.

He wants to change the tax code and direct government funding to reward heterosexual marriage.

ROGER SEVERINO: It stands to reason when you have a stable relationship, that tends to increase the birth rates.

And that's what the social science shows.

It cannot be just babies at all costs, right?

Babies aren't commodities.

It has to be in the context of family, which is marriage, work, stability, commitment.

That sort of vision is what leads to the maximum human flourishing.

SARAH VARNEY: What do you say to people who hear you in our leery of this movement, that somehow it's going to impose a sort of a set of particularly religious beliefs on a pluralistic society?

ROGER SEVERINO: Nonsense.

It's - - there's no coercion involved.

SARAH VARNEY: But Malcolm and Simone Collins say, in the future, women's equality will be at risk.

SIMONE COLLINS: If people who care about women's rights and people who care about feminism choose to not have their views represented in the future, and they won't be if they don't have kids and raise them in their culture well, we are going to lose those rights.

CHARLES HAYWOOD, Pronatalist: Men with family should be preferred in advancement in jobs and generally women should not have careers.

SARAH VARNEY: Charles Haywood is among those in the movement who believe women's demands for equality are responsible for declining fertility.

CHARLES HAYWOOD: The result over 60 years has been what we see now, which is that we have a lot fewer masculine men, and this, of course, is only one of the many contributors to the decline in birth rate, but I think it's a very important contributor to it.

SARAH VARNEY: Countries have long used state power to restrict women's rights, including banning contraception and abortion, and forcing pregnant women out of the work force.

LESLIE ROOT: When you generate a panic about birth rates, in the context of a society where reproductive labor is done primarily by women, this idea that this is women's job and the reason that birth rates are low is because women are not doing their job is really dangerous.

SARAH VARNEY: Back in suburban Maryland, Bethany Mandel knows having six kids is not the norm, but she says it's possible because of her equal partnership with her husband.

BETHANY MANDEL: That very trad misogynist voice, it's not unique to the pronatalist world.

It's a real problem on the right, and it's really disturbing, but the problem is in any sort of political discourse that you have, the craziest people are the loudest.

SARAH VARNEY: But does there need to be other voices in this movement that are kind of explicitly pro-women, that are kind of making the defense for women's equality and pushing back?

BETHANY MANDEL: Yes.

Yes, but the problem is, being more moderate, there's no fame, there's no money in that game.

And so it's not super appealing to spend your life advocating for a position that not many people listen to.

SARAH VARNEY: Mandel says, at the end of the day, she and her husband just want to spend time with their kids, but that leaves a vacuum for others to shape the future of the movement.

For "PBS News Hour," I'm Sarah Varney in Maryland.