PARIS (AFP) - Teenage fathers are more likely to have children with health problems because of mutations in their sperm, a study said on Wednesday.

Previous research had found that teens are likelier than 20- to 35-year-olds to father a child with autism, low IQ, schizophrenia or spina bifida, although the risk is low in absolute terms.

Delving into the phenomenon, researchers led by Dr Peter Forster at Britain's University of Cambridge analysed the DNA of more than 24,000 parents and their children in Germany, Austria, the Middle East and Africa. The youngest mother was 10.7 years old and the oldest 52.1 years, while the youngest father was 12.1 years and the oldest 70.1.

The sperm cells of teenage fathers had about six times more mutations than the egg cells of teenage mothers, the team found. They also had about 30 per cent more mutations than the sperm cells of 20-year-old fathers.

The study did not explore the health of the teenage dads' offspring, but the findings offer a strong explanation for the problems that have come to light, said Dr Forster.

Still unclear is why teen sperm mutations occur.

One reason could be that teenage sperm cells are "error-prone" when they replicate, said Dr Forster.

The findings were "no cause for panic", the researcher said by e-mail.

Overall, there are about 1.5 births with defects in the general population, and a 30 per cent increase "means that teenage fathers would expect to have 2 per cent of their children born with defects".

Evidence has accumulated in recent years of lower sperm quality in fathers 45 or older.

A study published in the journal JAMA Psychiatry in February last year said people whose fathers were 45 or older at the time of their birth were 25 times more likely to have bipolar disorder than those whose dads were from 20 to 24.

Children of older fathers were also 13 times more likely to have attention deficit hyperactivity disorder.

The best time, biologically, to be a father "is between 20 and 35 years of age", said Dr Forster.

"But even older fathers have only a modest increase in risk."