Transcript for The Biological Clock of Men and Having Babies
to a headline about modern health and the importance of a man's biological clock. Most of the emphasis through the years has been put on the age of a pregnant mother and risks for the child. But tonight, a giant, new study puts new emphasis on the age of the dad. And ABC's chief medical editor, Dr. Richard Besser is here. Rich? Reporter: The older Yo uh are when you have your children, the greater they are for mental illness. This was a huge study. It was very well done. It was in Sweden. They followed 2.5 million men. The age of the dad really matters when the problem they face. Mental illness, what do you mean? Reporter: Compared to the youngest fathers, those who were fathered by men older than 45, three-times the risk for autism. 13-times the risk for attention deficit disorder. 25-times the risk for bipolar. And twice the risk for schizophrenia. The risk was low. But your risk went up as your dads age. Men over 45. And any conjecture about why? Anything specific? Reporter: This doesn't prove it's age. But the older you are, the more mutations in sperm. That may be the link here. If you wait, there is a risk involved. Father and mothers, something to think about tonight. It was an emotional day, as
This transcript has been automatically generated and may not be 100% accurate.