Babies with a young teenage dad are a third more likely to suffer from a birth defect, new research has suggested. 
Using genetic data collected from more than 24,000 parents and their children, Cambridge University scientist Peter Forster and his colleagues, writing in the Royal Society Journal Proceedings B, show that the rate of genetic changes or mutations being passed on to babies born to fathers aged 15 and under is 30% higher than for men in their twenties and thirties.
This tallies with the finding that birth defects occur at the rate of about 1.5% in the general population but 2% in babies with teenage fathers.
"It's a 'j-shaped' curve," explains Forster. "The risks of passing on mutations are higher in the younger boys, drop again as they enter their twenties and then rise with increasing age after that." The reason for the findings, which were based on paternity tests on family groups seeking asylum in Germany, are not clear.
"There are two possibilities," says Forster. "One is that the precursor cells that make sperm have been dividing during boyhood in ways that we are unaware of, and this causes these extra mutations to occur. The other is that these same precursor cells were previously inactive but then suddenly have to start producing sperm and they are not quite ready yet and make more mistakes when they are copying their DNA."
Higher rates of mutation and birth defects were probably of little consequence historically, Forster speculates, because, for nutritional reasons, girls would not have become reproductively active until they were older and would have been more likely to couple with an older partner rather than a younger one.
But in the modern day, when individuals are better nourished and hence reproduction can occur from a younger age, this is potentially a more important consideration.
"It's a factor to bear in mind with teenage pregnancy," he says.
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