A total of 3,659,289 babies were born in the US in 2021, according to new data released this week by the CDC’s National Center for Health Statistics. That’s 1 percent more than the 3,613,647 babies born in the US in 2020, which means that the natal class of 2021 is now what passes for a baby boom in the United States of America.

Indeed, 2021 represents the first time since 2014 that the number of babies born in the US actually increased, bouncing back from a pandemic year in 2020 that saw the largest one-year drop in births in nearly 50 years.

The 2020 dip in births wasn’t much of a surprise to demographers. Despite the popular misconception that events like blizzards and blackouts that keep couples homebound inevitably lead to more babies nine months later, the lockdowns of early 2020 were not particularly conducive to the conceiving of children. Surveys conducted during 2020 found that as many as a third of American women changed their reproductive plans because of the pandemic, while and as many as half of American adults reported a decline in their sexual activity.

That 1 percent increase in births in 2021 stems in part from planned pregnancies delayed for a year or so, until the country saw improved epidemiological and economic conditions during the later stages of the pandemic, as stimulus and unemployment aid from the government made would-be parents a little less apprehensive about bringing a new life into the world.

So the playgrounds and preschools might be a smidge more crowded over the next few years. But make no mistake: This is not a baby boom that is meant to last.

A momentary bump amid a long-term decline?

Despite the 2021 bump, there were fewer babies born last year than in 2019 before the pandemic. Both the total number of births and the birth rate in the US have been on a general decline since 2007, when 4,316,233 members of Gen Z entered the world, the highest single number in US history.

The debate over precisely why US adults are having fewer or no children could fill several books. (Note to the babies of 2021: books are accumulations of paper affixed between two covers for the purposes of reading or, at the very least, showcasing in the background of your Zoom calls.) Economic insecurity — it’s not an accident that fertility decreases accelerated with the Great Recession — fears about climate or safety, reduced marriage rates, personal lifestyle choices; they each play a role.

But the more proximate causes of the birth decline in the US — and the reasons why it is extremely unlikely to change much in the future — can be found buried in the new data release.

Older and fewer

Even with the overall increase in 2021, one group of potential mothers in the US experienced an all-time low in birth rates: teenagers.

The provisional birth rate for females aged 15 to 19 was 14.4 births per 1,000 women, down 6 percent from 2020 and the lowest level on record. 2021 continued a historic decline in teen births, which have fallen 77 percent since the most recent peak in 1991, and has declined an average of 7 percent annually since 2007. At the same time, the birth rate for women ages 35-39 and 40-44 rose 5 percent and 3 percent in 2021, respectively.

This is largely a good thing! The drastic reduction in teen birth rates is one of the biggest social policy successes of the past quarter century. Children of teen mothers are at higher risk of being born prematurely and underperforming in school, while teen parents are less likely to get a high school diploma or finish college. As childbearing is delayed to the 30s and beyond, parents are likely in a better position to care for the children that they choose to have.

But delayed childbearing has its demographic downsides. The longer would-be parents choose to wait — or are forced to wait because of difficulty finding a partner or becoming financially settled — the fewer children they’re likely to have over the course of their lifetimes. Planned larger families shrink in reality, and more people end up not having children whether they wanted to or not.

Grayer and grayer

And that’s exactly what’s been happening.

While the total fertility rate — the average number of babies that will be born to an average woman over the course of her lifetime — rose slightly to 1.66 in 2021 from the year before, that’s less than half what it was in 1960, and well below the population replacement level of 2.1, which the US hasn’t surpassed since 2007.

By 2034, according to Census Bureau projections, there will be more adults over 65 than children for the first time in US history. While the US population will keep growing, passing 400 million by 2058, it’ll do so with decreasing speed, and the number of working-age people will fall well before that.

And the US has it good compared to most other countries, demographically speaking. The birth rate in Italy hit a 160-year low last year, while in South Korea — where the population has been declining for more than two years — total fertility rate hit a record low of 0.86 for the first quarter of 2022. From the family-friendly Scandinavian countries to Latin America and the Caribbean to even China, still the most populous country on Earth, birth rates are sliding downwards, even as global population is set to pass 8 billion soon.

The US has the advantage of being able to offset declining births by bringing in more people from abroad. Just one problem: immigration to the US fell off a cliff beginning with the Trump administration, and hasn’t recovered much under President Biden. Altogether there are 1.8 million fewer working-age immigrants in the US now than there would have been had pre-2020 trends held — potential workers who would be pretty useful in a tight labor market with high inflation.

The 2021 uptick in births doesn’t look like much of a baby bump against what used to be the norm in the country’s not-too-distant past. But compared to what’s likely to come, it’s a veritable boom.

A version of this story was initially published in the Future Perfect newsletter. Sign up here to subscribe!