'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. Men gain weight after they become fathers for the first time whether or not they live with their children, reports a large, new Northwestern Medicine study that tracked the weight of more than 10,000 men from adolescence to young adulthood. 
 
 The typical 6-foot-tall man who lives with his child gained an average of about 4.4 pounds after becoming a first-time dad; the 6-foot-tall dad who does not live with his child gained about 3.3 pounds, the study reports. That's a 2.6 percent rise in BMI (body mass index) for resident dads and a 2 percent rise in BMI for non-resident dads after controlling for other variables. 
 
 By contrast, the average 6-foot-tall man in this group who was not a father actually lost 1.4 pounds over the same time period. 
 
 This is one of the first studies to examine how fatherhood affects a major biomarker of health, the BMI. The study will be published July 21 in the American Journal of Men's Health. 
 
 The study controlled for other factors that could contribute to weight gain such as age, race, education, income, daily activity, screen time and marriage status. It is already known marriage results in a weight gain for men. The fatherhood weight gain is in addition to the increase resulting from marriage. 
 
 "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. "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." 
 
 New fathers' weight gain may be due to changes in lifestyle and eating habits. 
 
 "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 said. "Your family becomes the priority." 
 
 Eating habits may shift as the house fills with cookies, ice cream and other snack food. 
 
 "We all know dads who clean their kids' plates after every meal," Garfield said. His personal weakness: finishing his kids' leftover cheese pizza. 
 
 Over the 20 years of the study, all 10,253 participants had their BMI measured at four different time points: early adolescence, later adolescence, mid-20s and early 30s. Each participant was categorized either as a non-father, resident father or non-resident father. Then researchers looked at each person's BMI at each time point and took the average of all those measurements to determine whether their fatherhood status was associated with their BMI. 
 
 Because many new dads don't have a personal physician, pediatricians are in a good position to counsel dads about taking care of their health. Many new fathers think they're too young and healthy to need their own doctor, Garfield said. 
 
 "New dads are coming into the health care system as a pediatric chaperone," Garfield said. "This is an opportunity to talk about things that are important for dad's health and the child's health and to offer dads nutritional counseling and mental health education." 
 
 Previous research by Garfield showed new fathers have an increase in depression symptoms in the early years after their child's birth. 
 
 "We now realize the transition to fatherhood is an important developmental life stage for men's health," Garfield said. "It's a magical moment where so many things change in a man's life. Now the medical field needs to think about how can we help these men of child-rearing age who often don't come to the doctor's office for themselves." 
 
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 Other Northwestern authors are Joshua Rutsohn, Thomas W. McDade, Emma K. Adam and P. Lindsay Chase-Lansdale. 
 
 The study is titled "Longitudinal Study of Body Mass Index in Young Males and the Transition to Fatherhood." 
 
 The study was supported by grant K23HD060664 from the Eunice Kennedy Shriver National Institute of Child Health and Human Development of the National Institutes of Health. ||||| According to Clemson student Mackenzie Pearson, who wrote a viral essay essay on the appeal of the dad bod, it’s a physique that’s a “nice balance between a beer gut and working out,” the result of going to gym but indulging in a few pizzas once in a while and being okay with that. (Think John Hamm, and Chris Pratt before he went Jurassic.) 
 
 And according to scientists, Pearson and her demographic have pretty much nailed it. The source of that “more human, natural and attractive” body is unique to fathers and can be traced to simply having kids. 
 
 In a study published in the American Journal of Men’s Health, Dr. Craig Garfield, a pediatrician at Northwestern University Feinberg School of Medicine and his colleagues dove into a database of 10,263 men beginning when they were 12 years old and followed them for up to 20 years. 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. 
 
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 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. 
 
 “It’s a unique look at the influence that a social phenomenon, becoming a father, has on a biological marker, namely BMI,” says Garfield. “It really plants fatherhood as a potential social determinant of health for men.” 
 
 That’s a critical finding, especially since men, and in particular young men, are typically less proactive about taking care of their health. Garfield notes that while many men will quit smoking and drink less and otherwise try to become healthier when they become fathers, there may be other factors associated with caring for kids that counteract those good intentions, such as being surrounded by more kid-friendly, high calorie foods and snacks, as well as their leftovers. 
 
 “From my own point of view, we wouldn’t have as many pizzas in the house if the kids weren’t around, and we wouldn’t have the brownies my wife makes if the kids weren’t around,” says Garfield. “Having kids around changes not only the food in the house and what is available to you for meal, but also for snacks. It also changes whether you are able to find time to get out and exercise and get enough sleep and take care of yourself.” 
 
 Read more Dadbod, Mombod and Our Pretty Bad Bod Prob 
 
 Dads, of course, are not alone in experiencing these effects of parenthood. But this is the first study to tease out specifically the effects of fatherhood on weight gain over time. Since men are less likely to be seeing doctors regularly, if they are joining their partners during prenatal visits or pediatric visits, says Garfield, those are good opportunities to talk to them about their own eating, exercise and sleep habits to make them aware of the sneaky way that pounds can creep up on dads and potentially affect their health (even if the look seems to have its own kind of physical appeal). 
 
 Read next: For the Dad Who Is Confident About How He Looks in Swim Trunks 
 
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– 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.)