"Men are also at the mercy of age when it comes to having kids."

The male body constantly makes new sperm that is capable of producing a child...

... but they can contain dangerous mutations. "As men get older, maybe there is some sperm available, but a lot of that DNA may be abnormal," says Harry Fisch, author of the pioneering 2004 book "The Male Biological Clock." "After you make so many copies, the print may not be so useful."

Data are scarce on trends in paternal age, which perhaps explains why the correlation between paternal age and birth defects went undetected for so long. And, of course, "nobody likes to think that they're aging," says Dr. Fisch. "Certainly men. They were on the throne, they were the kings: 'We don't age, we stay fertile longer than women, we can have babies into our 90s....' Men live in denial."

Don't be cocky.