A new study has shown that a father’s age plays a role in pregnancy outcomes, with harmful genetic mutations becoming more common in the sperm of older men.

A report in the journal Nature stated that researchers also discovered that the increase in mutations is due to a subtle form of natural selection.

Some mutations gain a competitive advantage during sperm production in the testes, alongside the continuous accumulation of random DNA changes as men age.

The researchers found that among 81 healthy volunteers, about 2% of sperm from men in their early 30s carried disease-causing mutations, compared to between 3% and 5% in men aged 43 to 74, and 4.5% in men aged 75.

The researchers noted that some mutations have previously been linked to cell growth and development, while others are associated with severe neurodevelopmental disorders in children and hereditary cancer risk.

They also said that some mutations may impair fertility and embryo development or lead to pregnancy loss.

Matt Hurles, a professor at the Wellcome Sanger Institute in Hinxton, England, said in a statement, “Not only do some DNA changes persist, but they also grow within the testes, meaning fathers who have children later in life may unknowingly be more likely to pass on harmful mutations to their offspring.”

In a complementary study involving more than 54,000 parents and their children and 800,000 healthy individuals, also published in Nature, some researchers analyzed mutations that had actually been passed on to children rather than those measured directly in sperm.

They identified more than 30 genetic factors where mutations give sperm cells a competitive advantage through natural selection, including many linked to rare developmental disorders and cancer. Many of these mutations overlap with genetic factors directly observed in sperm.

The researchers said this work highlights how natural selection can be directly observed within sperm DNA in children, increasing the likelihood of inheriting certain genetic disorders.

Hurles, who co-authored both studies, said, “Our findings reveal a hidden genetic risk that increases with the father’s age.”