In modern society, more and more women are ignoring their biological clocks and having babies whenever suits us best, but new research has shown that our genes do still play a role in when we have our first babies. In fact, human females are actually evolving to become fertile younger and younger.

After analysing the DNA of almost 7,000 women in the UK and the Netherlands, researchers have found that around 15 percent of the variation in the age women have their first child, and 10 percent of the difference in the number of children they have, can be attributed to genetics.

This is the first study to look at both unrelated women as well as female twins, and it aimed to tease apart exactly why women are now having babies on average four to five years later today than in 1970, with the average age of first-time mothers now between 28 and 29.

"What we see in this study is a clear genetic component linked to the age of mothers when they have their first child, and to the number of children they have," lead researcher Melinda Mills from Oxford University in the UK told Ian Sample over at The Guardian.

But this genetic component should actually be making us have children earlier and earlier, and so the fact that we're delaying the process can only be attributed to societal factors, the team explains.

"Although genes play a significant part, it seems wider social changes, such as an expansion of women in further education and work, as well as the availability of effective contraception, are having a stronger effect on determining when women in modern societies have children," co-author Felix Tropf from the University of Groningen in the Netherlands said in a press release.

To work out how genes influence fertility, the researchers looked at 4,300 unrelated women in the Netherlands, and 2,400 females who have a twin in the UK, and then studied their DNA and whether/when they had children, as well as how many they had. The results showed that there are a series of genes that influence how young a woman has a child, and how many she has.

Interestingly, they also found an overlap between the two sets of genes, and showed that women who have their first children earlier are also more likely to have more children. So the genes for early fertility will be passed down more often.

"Natural selection is not just an historical process. Modern societies are still evolving today, with early fertility patterns being an inherited reproductive advantage," the release explains. Obviously, this would mean that if we were all listening to our genetic urges, we'd be having children younger and younger, which is the opposite of what's actually happening.

While this all sounds like another kick in the guts for women who don't want to pop out a baby at their body's ideal age of sweet 16, there is actually a little hope to be gleaned from the research. If our genes do play a role in the age we're most fertile, then women may one day be able to test their DNA to find out exactly how long they can delay having a baby, as well as the optimum time for them to start trying.

"This research tells us there are genetic differences between women which could be significant for women making decisions about when to have their first baby," Mills told The Guardian.

The findings of the research have been published in PLOS ONEThe team is now researching more about the exact genes that are involved in this process, and is already planning a follow-up paper soon.