It’s called “dad bod,” and a recent scientific study says it’s a very real phenomenon.

A study by Northwestern University’s Feinberg School of Medicine published this week in the American Journal of Men’s Health found that men who become fathers gain weight and see an increase in their body mass index, which is a measurement of body fat based on their weight and their height, according to a Washington Post report.

In the study, which tracked 10,000 men over the course of 20 years, men who didn’t become dads actually seemed to lose weight over that period.

It’s the first time a study has taken a deep look at the phenomenon of “dad bod,” as well as the BMIs of young men compared to fathers, and it shows that fatherhood can really affect the health of those men. The more weight those fathers gain and the higher the BMI, the more risk they have of developing health problems like heart disease or even cancer, said Craig Garfield, an associate professor at Northwestern, according to the report.

The study found that first-time resident dads — dads who live with their children — had their BMIs increase by 2.6, compared to non-resident dads, who only increased by 2 percent. That’s a difference of 4.4 pounds for a dad who is six feet tall and lives with his kids and a 3.3 pound weight gain for a non-resident dad. And a man in similar circumstances who didn’t have a child actually lost 1.4 pounds.

So why the increase in BMI? It probably has to do with lifestyle changes. Once you have kids, your responsibilities shift, and therefore you may not prioritize exercise and staying in shape in general.