"Fatherhood can affect the health of young men, above the already known effect of marriage," said lead author Dr. Craig Garfield, associate professor of pediatrics and of medical social sciences at Northwestern University Feinberg School of Medicine and attending pediatrician at Ann & Robert H. Lurie Children's Hospital of Chicago. They looked specifically at how body mass index (BMI), a combination of height and weight, changed over time as the men either became fathers or did not, and for those who did, whether they were what the researchers called resident fathers who lived with their children, or non-residents who lived separately. View Sample Sign Up Now 
 
 Read more Why We Accept ‘Dad Bod’ on Rich Men 
 
 Whether or not they lived with their kids, becoming a father was linked to around a four pound increase in weight over the study period, while remaining child-free was associated with a 1.4 pound weight loss for a six-foot-tall man. “Having kids around changes not only the food in the house and what is available to you for meal, but also for snacks. 'Fatherhood effect' is average weight gain of 3.5 to 4.5 pounds 
 
 Non-fathers lose weight during same time period 
 
 Key time for pediatricians to counsel fathers, who often don't have their own doctors 
 
 CHICAGO --- All those leftover pizza crusts you snatch from your kids' plates add up.
– There's science behind the plump dad bod: A new study that tracked the weight of 10,000 males over 20 years from adolescence into their early 30s finds the average 6-foot tall, first-time father gains 4.4 pounds if he lives with his child after birth and about 3.3 pounds if they live separately. That equals a 2.6% and 2% jump in a man’s BMI, respectively, according to a Northwestern University release. And men, don't try to pass off the weight as a side effect of marriage. Though a recent study found marriage does add a little to your waistline, the fatherhood weight gain is a separate issue "above the already known effect of marriage," says lead author Dr. Craig Garfield. Regardless of marital status, childless men actually lost weight in the same period: about 1.4 pounds. So why are men gaining weight while chasing after little ones? It likely comes down to a change in eating habits and lifestyle, researchers say. "You have new responsibilities when you have your kids and may not have time to take care of yourself the way you once did in terms of exercise," Garfield says. While many men quit smoking or cut back on alcohol when they have children, there may be more snacks and leftovers lying around, he tells Time. "We all know dads who clean their kids' plates after every meal." As first-time fathers often feel they're too young to have their own doctor, Garfield suggests pediatricians strike up conversations with child-accompanying dads about their own health, since "the more weight the fathers gain and the higher their BMI, the greater risk they have for developing heart disease as well as diabetes and cancer." (Here's how much weight marriage packs on.)