For the first time in human history, American women in their 30s are having more babies than women in their 20s, with the average age of first-time mothers sitting at about 28 - a massive two-year leap from 2014.

Not only is this a stark reminder of just how quickly our species can change in response to societal trends and pressures, it also contradicts conventional wisdom stating that later pregnancies are riskier - despite the increase in older mums, the infant mortality rate in the US remains stable.

The new figures, released by the US Centres of Disease Control and Prevention (CDC), are based on an initial review of birth and death certificates filed across the US last year, and the CDC is expected to publish a full analysis in the coming months.

But even before that analysis is released, we can glean a lot from the raw data, which states that the birth rate for women aged 30 to 34 was 102.6 per 1,000, and the rate for women aged 25 to 29 was 101.9 per 1,000.

That might not sound like much of a difference, but just 12 months prior, the birth rate for women aged 30 to 34 was 101.5 births per 1,000, whereas women aged 25 to 29 were at 104.3 births per 1,000.

Now that the 30-year-olds have overtaken the 20-somethings, American mothers have broken a 30-year trend that had women in their late 20s as the group with the highest birth rate. 

Another trend that likely helped the birth rate favour older mothers is the fact that teen pregnancies are becoming less common, with first-time births among teens dropping by almost half between 2000 and 2014.

As Katelyn Harrop reports for Attn: "In 2016, there were less than 25 births for every 100,000 teenage females in the United States. That's a serious drop from the late 1990s, when there were 93 pregnancies for every 100,000 teen girls."

Here are some more insights from the initial report:

  • Births for women older than 34 also increased, with 52.6 births for every 1,000 women in 2016 compared to 51.8 births per 1,000 women in 2015
  • The average age for first-time mums is now roughly 28 - as recently as 2014, that average was 26.3
  • The teen birth rate continued to drop in 2016
  • The infant mortality rate stayed about the same.

While these figures only apply to American women, they reflect a growing trend away from the 'taboo' of having babies later in life.

Any childless woman over 30 will be all too familiar with the risks of delaying pregnancy, but research is starting to show that the benefits of having a child when you're better established in life can actually outweigh the biological complications that can come from an older pregnancy.

A study of 1.5 million men and women in Sweden last year found that when mothers decided to delay having kids until they were older - even into their 40s - they were more likely to have children who were taller, more physically fit, got better grades in high school, and were more likely to go to university. 

The reasoning goes that while the physical risks of having a baby later in life are still very real, women in their 30s are more likely to be financially stable, so can offer their children better healthcare and education options than women in their 20s.

"The benefits associated with being born in a later year outweigh the individual risk factors arising from being born to an older mother," lead researcher Mikko Myrskylä from the Max Planck Institute in Germany said at the time.

"We need to develop a different perspective on advanced maternal age. Expectant parents are typically well aware of the risks associated with late pregnancy, but they are less aware of the positive effects."

What was also really interesting is how quickly things can change for children born of the same mother: the study found that a child born when their mother was in her 40s ended up being better educated than the child born in her 20s - her 'fertile prime'.

"Those 20 years make a huge difference," said Myrskylä. "A child born in 1990, for example, had a much higher probability of going to a college or university than somebody born 20 years earlier."

Consider how different things are going to be for babies born two decades from now, with gene-editing technology set to revolutionise how we conceive and prevent genetic disease before our children are even born.

So while there are pros and cons no matter when you have your baby, it looks like women - in the US at least - are delaying pregnancy later than ever before.

And if that means we can focus on making the world a better place for the next generation, that's a pretty great result for our species.

The CDC data has been published here.