American scientists have confirmed that men get fatter when they have children.

The new research, published in the American Journal of Men's Health, looked at body mass index (BMI) in young American men as they went from adolescence to fatherhood.

The study was carried out over 20 years and looked at over 10,000 men.

New fathers put on weight on entering fatherhood for the first time because they were less active outside the home, and because they spent more time in front of screens.

They may gain averages of 3.3 or 4.4 pounds, beginning in the first year of fatherhood.

Men’s weight is an important risk factor for diabetes, heart disease, cancer, and premature death.

The term dad bod appeared earlier this year and is described as 'a nice balance between a beer gut and working out'. 

Hollywood dads Adam Sandler and Vince Vaughan are often touted as examples of the body type.

Sunny Sandler, Adam Sandler, Sadie Sandler, and Jackie Sandler Photo: Matt Sayles / AP/Press Association Images

We already know that getting old causes men to put on weight, but to date there has been very little research on the way new fathers put on weight.

The authors say it means we should be talking to teenagers - before they have children - about healthy eating if we want to prevent obesity.

Graph showing standardised bmi levels before and after entering fatherhood

Whether or not a man is married, and whether or not he lives in the home with the child, also makes a difference.

Dads who are living with their kids will start at a higher BMI and will experience the same kind of increases a dad who doesn't live with his baby experiences.

Previous studies have suggested that men who are happily married are already likely to put on weight, though being married also has health benefits for the man. 

High numbers of dads who are living with their babies are also married.

Between the ages of 23 and 29 men who do not become fathers will likely lose weight, as their counterparts with children put weight on.