U.S. fertility is not recovering from the financial crisis — and demographers aren’t sure why.

The fertility rate fell to a record low 62.9 births per 1,000 women aged 15-44 in 2013, according to the National Center for Health Statistics.

The total number of births, at 3.96 million, inched up by a mere 4,000 from 2012, the first increase since the financial crisis. But the total fertility rate, or TFR, the average number of children a woman would have during her child-bearing years, fell to just 1.86, the lowest rate in 27 years. TFR is considered the best metric of fertility. A TFR of 2.1 represents a stable population, with children replacing parents as they die off.

Demographers expected the fertility rate to fall during recession, as financially strapped families put off childbearing. But what has surprised some demographers is both the depth of the decline and the fact that fertility has continued to drop even over the course of the country's five years of slow but steady recovery. The rate has fallen steadily each year since 2007, when it stood at 2.1 percent.

The result: There would have been at least 1.3 million more births during the recession and recovery if fertility had remained at 2007 levels, calculated Kenneth Johnson, a demographer at the University of New Hampshire.

Johnson noted that the excess of births over deaths is now the lowest it has been in 35 years. There were more deaths than births in a third of U.S. counties last year. “More people died than were born in Maine for the second year in a row and West Virginia again had more deaths than births as it has for a number of years,” Johnson wrote.

Why the decline in fertility has persisted even as the worst effects of the labor market have abated is “an excellent question that no one knows the answer to,” according to Johnson.

One foreseen factor behind the dropoff in childbearing is the rapid decline in Hispanic-American fertility.

For several decades, high Hispanic childbearing has been driving U.S. population growth. White fertility has been under the 2.1 replacement rate for decades, and ranged from 1.7 to 1.9 in the 2000s. The TFR for black Americans first fell below 2.1 in the early 2000s.

But the number of children per Hispanic-American woman has plummeted from just under three in 1990 and 2.7 as recently as 2008 to 2.19 in 2012, just above the replacement rate.

That decline has been mirrored in other Hispanic countries. Mexico’s TFR has fallen precipitously, from 6.7 in 1970 to 2.2 in 2012, according to the World Bank. A similar decline has taken place in El Salvador, and to a lesser extent Honduras and Guatemala, all three prominent countries of origin for American Hispanics.

But part of the post-recession drop-off in U.S. fertility was not as anticipated, namely, the dramatic shift in fertility from younger to older women.

While birth rates for women in their teens and early 20s have fallen to record lows, the corresponding rates for women in their 30s and 40s have continued to rise, uninterrupted, throughout the recession. The average age of women at their first birthhas risen for four straight decades.

Mark Mather, an associate vice president at the Population Reference Bureau, explained that the shift to older childbearing largely reflects young adults waiting to get married. That entails fewer children overall, Mather noted, because “it does get more difficult as people wait until they’re older to start families.”

The big question is whether fertility rates for young women will pick up as the economy gathers strength. “Uncertainty still kind of rules the day,” said Sam Sturgeon, the president of Demographic Intelligence, a firm that provides demographic forecasts for businesses.

Sturgeon's own expectation is that young women will return to childbearing once the economy strengthens, because “there’s a very strong two-child norm in the U.S., and we haven’t seen any evidence that women want fewer.” Nevertheless, he said, he wouldn’t be surprised if fertility rates for teens and women in their 20s did not pick back up.

It’s less likely that the U.S. will return to above-replacement fertility, a benchmark that most advanced economies have fallen well below in recent decades. Sturgeon pointed out that the U.S. only reached a TFR above 2.1 in 2006 and 2007, at the height of the housing bubble years.

The consequences of America’s recession baby bust are already significant. “We’re getting to the point where it’s dropped far enough and for a long enough period of time that it’s going to have serious implications” for the population and the economy, Mather said. With declining fertility, the U.S. population would age, and ultimately the labor force would decline as older workers retire — a trend already well underway with the Baby Boom generation reaching their mid-60s.

The financial crisis “has had the most punishing impact on demographic trends of anything since the Great Depression,” Johnson said.