Child poverty in the U.S. has more than doubled in a year, and we have a pretty clear idea what drove it: Congress let the expanded child tax credit expire.

It's rare for a government policy to have an immediate and measurable impact on an individual or large portion of the population. But experts say the monthly payments to low-income families with children were doing just that.

After the expanded credit took effect, child poverty hit a historic low of 5.2% a year ago. New Census data shows it has since rocketed to 12.4%.

Doctors are seeing this play out in real time.

Who did we talk to? Pediatrician and researcher Megan Sandel, who treats kids at Boston Medical Center.

NPR spoke to her a couple of years ago while the monthly payments were still going out to families. Here's what she said at the time:

I really have to call out the child tax credit. We have seen in the last six months families starting to get back on their feet. We have started to graduate kids from our Grow Clinic, finally. And a lot of that has to do with being able to have that consistent check every month that they know they're getting.

And here's what Sandel told All Things Considered's Ari Shapiro this week:

We're seeing families just under that enormous stress again. They are having to make really tough decisions. They have kids going back to school, and they don't know if they can afford a backpack and that school uniform, and needing to make really difficult choices about whether or not they're going to be able to actually be able to afford the food that their kids need to grow.

Want to learn more? Listen to the Consider This episode on how families are sliding back into poverty.

What's the context?

• As All Things Considered reported, in 2021, Congress increased the amount of the child tax credit as part of the American Rescue Plan. It also expanded eligibility to include millions more low-income families.
• Experts and parents reported measurable relief, but the move was temporary and wasn't renewed.
• The recent rapid rise in child poverty coincided with other factors — like record inflation — but experts say the end of the expanded child tax credit was a key factor.

What is Sandel seeing now?

Sandel says she is most concerned about stunted growth, weight loss and poor performance in school among the kids she treats.

What we're starting to see is kids flatlining, kids who should be growing, should be gaining weight, should be, frankly, growing the brain that they need for the rest of their lives. And we're seeing kids not grow. We're seeing kids lose weight. Which when you're 3 or 4 years old, that is a medical emergency. What's going on? And a lot of times when we really dig deeper, it's simply because people can't afford enough food and are stretching beyond what they can deal with.

Sandel does call out inflation and the rising cost of housing for adding an additional burden to already struggling families. But she says effective policy can help families navigate those factors.

And so what I don't want people to walk away from is to say, "Oh, well, inflation, it doesn't matter if you give people more money, it's just going to be spent and it won't travel as far." I do think that in many ways, it really is about the positive effects of putting money in people's pockets.

How does this make her feel as a pediatrician?

Mostly, Sandel says she doesn't understand why the policy was allowed to expire.

We have something that worked really, really well. And so I want to ask, what are the ways in which, you know, we can say to ourselves, this is worthy of investment? Because what I like to say is I can do my best role as a physician to help kids grow. But what I need is policymakers to do their job to be able to help kids grow, too. And that is really in their hands.

So, what now?

• Sandel says she is not ready to stop fighting for policies to help kids and families, adding that the new child poverty rates are a "wake-up call" for all involved: "I'd love to be able to come on in a year and be able to talk about that we got the number back down to 5% and beyond."
• And as Ludden reports, the child poverty rates have also fueled political debate over bringing back an expanded child tax credit — although it's been at a stalemate in Congress.

Learn more:

• Child poverty more than doubles — a year after hitting record low, Census data shows
• Poverty and uninsured rates drop, thanks to pandemic-era policies
• How poverty makes workers less productive