NEW YORK, N.Y. (WPVI) -- Many men gain a new sense of responsibility and purpose when they become fathers. A new study suggests they also gain 3 to 5 pounds.
The research wasn't designed to prove fatherhood causes weight gain and raises more questions than it answers.
But one outside expert, while noting its limitations, said the research is provocative and should spark further study.
Doctors pay attention to the weight gain of mothers - both before and after pregnancy.
But the waistline of dads? That's not on most doctors' radar, said Tom Wadden, director of the University of Pennsylvania's Center for Weight and Eating Disorders.
The study's lead author - Dr. Craig Garfield of Northwestern University - said he could only speculate about what's behind the extra pounds.
"For men who become fathers, their whole life changes," Garfield said.
They may sleep less, exercise less, and experience more stress - all of which can lead to weight gain, he said.
It doesn't help that the food selection at home may gradually change to include more things like "making chocolate chip cookies with the kids," said Garfield.
A dad himself, Garfield said his weakness is finishing his kids' leftover cheese pizza.
Jamie Engles echoes that message.
"There were a lot of changes in my eating habits," he said.
"If there were any leftovers that he did not finish eating, rather than wasting or anything else, we would do a little snacking ourselves."
For their work, the researchers looked at results from another study, which tracked the health of adolescents over two decades.
The researchers focused on teen boys and young men, comparing weight changes in the 3,400 who became dads and the 6,800 who didn't.
There was a difference.
After becoming a first-time dad, a typical 6-foot-tall man who lives with his child will gain an average of about 4 pounds, the study suggested.
A same-sized man who does not live with his child can expect to gain nearly 3 pounds,
But a 6-foot man who does not have children typically loses about 1 pounds over the same time period, researchers found - after making statistical adjustments to iron out the potential influences of age, marriage, and other factors.
The study checked weights of the men at four times over the two decades.
The researchers were not able to determine at what point in time dads put on the weight.
Most of it could have gone on during the pregnancy, Wadden noted.
Registered dietitian Maya Feller urges parents not to get in a rut of mindless eating.
"Parents need to think about their health the same way that they think about their kids' health," she says.
The study found men who lived with their children were a little heavier to begin with, on average, and ended up heavier than the absent fathers and the men who didn't have kids.
Nearly three-quarters of U.S. men are overweight or obese, according to government statistics.
The study was published online Tuesday by the American Journal of Men's Health.