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A new study from the Centers for Disease Control and Prevention (CDC) shows that the average age of first-time mothers in the US is on the rise.

More US women are waiting longer to become mothers for the first time, according to figures released on Thursday.

Although that's a continuation of a trend that's been seen since as far back as the 1970s, lead study author TJ Mathews says that they saw something interesting happening in the data starting in 2009.

"There's more of an increase just in the last five years," he says. "To see something increasing steadily, it doesn't ever drop, and then to see it increasing even more is fascinating."

In the new report, researchers found that as of 2000, the mean age of new moms in the US was 24.9 years. That figure rose to 26.3 years by 2014, and the majority of that spike occurred after 2009.

Ken Johnson, a demographer at the University of New Hampshire, says this has everything to do with the so-called Great Recession, and on women delaying childbirth as a result of tough economic times.

"The other time in American history when there's been this kind of change was during the Great Depression itself," he says.

According to the new CDC report, women under 20 account for most of the change in behaviour. While Mathews and his team also saw an uptick in the number of first-time mothers who are over the age of 30, the more dramatic effect took place in the slowing rate of teenage pregnancy.

"This has been the great success story for the past number of years now," says Melissa Kearney, a professor of economics at the University of Maryland. "Teenagers are more likely to abstain from sex and those who are sexually active are more likely to use contraceptives."

Image caption The decline in teenage pregnancies is contributing to the changes, the CDC says

But the data also raises lots of questions about the demographic shifts that could lie ahead. Mathews says that the older women are at the time of their first birth, the more likely it is that they will have smaller families.

"You're less likely to have three, four, five births," he says.

Johnsons wonders whether the post-2009 delay in child births is a permanent effect, or one that will wear off as the economy improves.

"One of the big questions in this rising age of first-births is, are those births ever going to be made up, or are we going to not have those births at all?" he says. "Many demographers have been expecting the birth rates would turn up. That hasn't happened yet."

Not 'opting out'

Johnson estimates the recession could be responsible for as many as three million fewer births. On the other hand, should women eventually have just as many children as they would have before the recession, Johnson says the US could see a population "boom-lette".

One explanation of the higher first-time birth age to be sceptical of, says Ohio State University economic profession Bruce Weinberg, is that older, more educated and highly skilled women are increasingly "opting-out" of childbirth altogether.

"A lot of women are having multiple births, suggesting that they're turning to fertility treatments more for the highly skilled population," he says. "This phenomenon is not a phenomenon of highly skilled women increasingly delaying fertility. This is really about women not having children as teens."