By Maggie Fox

Here's another reason to discourage teen pregnancy: teenage fathers pass along six times as many genetic mutations to their kids as do teenage mothers. 

Researchers who looked at the DNA of more than 24,000 parents and their children found that when the father was 20 or younger, the children had many more mutations than did children of older dads, as well as many more mutations than the offspring of teen mothers and adult dads.

It might be that there's something wrong with the sperm of younger men and boys, says Dr. Peter Forster of the University of Cambridge in Britain and the Institute for Forensic Genetics in Munster, Germany, who led the study.

"The mutation rate is high in teenagers. It goes lower in the young adults and then rises again after 35," Forster told NBC News.

"The mutation rate is high in teenagers. It goes lower in the young adults and then rises again after 35."

The finding may help explain why birth defects are more common among the children of very young parents, Forster said, although he noted that the overall risk of defects is very low.

"It increases the risk by 30 percent," he said. "For the individual, that means an increase from a 1.5 percent risk of having baby with birth defects to 2 percent, so the individual teenager shouldn't be too worried," he added. "However, for policymakers an increase in birth defects of half a percent across the population is a serious matter, and policymakers should continue to discourage teenage parenthood."

The study used an unusual group of parents: Austrian and German parents who were having paternity testing done, as well as immigrants to Britain from Africa and the Middle East who were seeking to prove family members were really related to them.

Some of the parents were very young. The youngest mother was 10 and the youngest father was 12. The oldest mother at time of conception was 52 and the oldest father was 70.

They looked for mutations in the children that weren't found in either parent. There were not very many, but 54 were traced to the mother and 297 to the father. When they broke these mutation rates down by age, they found that teenage fathers were more likely to pass along mutations than all but the oldest fathers.

It's well known that older fathers and mothers alike are more likely to pass along genetic defects to their offspring, from Down's syndrome to dwarfism. Forster's team didn't look at the types of genetic changes linked with these conditions, but at a part of the DNA called microsatellites. These microsatellites change every time a cell divides and can tell you just how many divisions there have been since a cell first split into two more cells.

"The fathers' mutation rate then rises only slightly by age 50."

Forster believes that whatever is going on in passing along these mutations first happens in the so-called germ cells — the eggs and sperm. Girls carry all the eggs they'll ever have from birth, while males make fresh sperm all along. But it's possible the mutations arise in sperm precursor cells, which may not be made anew but which may exist from early childhood, Forster said.

The mutation rates they found correlate with the idea of a female steady supply of eggs, but they don't match the idea that sperm are always made fresh. Otherwise, males should have higher mutation rates all along.

Instead, they start out high and drop as the teens hit their 20s, starting back up again at about age 35.

"The fathers' mutation rate then rises only slightly by age 50," the researchers wrote in the Proceedings of the Royal Society B, a British scientific journal.

The next step will be to look at other types of DNA mutations in the children of teenage fathers specifically, and to see if they can be linked to specific birth defects, Forster's team said.