If you've put off starting a family, you're not alone.
In 1972, the average age of fathers of newborns in the U.S. was 27. Now, it's closer to 31 years old (30.9 years to be specific), the study finds.
If you need a little reassurance that it's not too late, consider this: Twice as many dads of newborns are now in the 40-plus age group, compared to the 1970s.
Of the roughly 4 million births each year in the U.S., about 9 percent of fathers are over 40. And "about 40,000 newborns have a father over the age of 50," says Michael Eisenberg, an assistant professor of urology at Stanford University Medical Center and the senior author of the study. "This surprised me," Eisenberg told us.
Eisenberg says there are potential risks and benefits of becoming a parent later in life. Typically, older mothers and fathers are farther along in their education and in their careers, "so with that comes more resources," Eisenberg says.
But for men, the risks are less clear.
As men age, studies show that the quality of their semen declines, but "the effects of advanced paternal age are incompletely understood," as another study points out.
Some reports have found "increased risk of autism, psychiatric illness, neurologic disease ... and chromosomal abnormalities in children born to older fathers," writes Eisenberg and his coauthors in the manuscript.
Eisenberg says here's how he helps his patients think about the potential risks. He explains the absolute risk of having a child that develops one of these conditions is unlikely to be changed that much by being an older father. Your risk may be higher, but you've got to put that risk in context.
"It's kind of like buying two lottery tickets, instead of one," he says. "If you buy two, you're chance of winning doubles," but you're still very unlikely to hit the numbers.
The U.S. isn't alone when it comes to the rise in paternal and maternal ages. In Germany, the median age of fathers has risen to 33. And, in England, fathers aged 35 to 54 account for 40 percent of live births , according to this 2006 study.