Like mothers after giving birth, fathers also gain weight and develop an increased body mass index (BMI), says a new study from researchers of the Northwestern University Feinberg School of Medicine. 

Researchers studied the data of 10,000 men during their adolescence until adulthood for a 20-year period and their BMI was measured at least four times during the course of the study. Of the participants, a third or 3,425 became fathers.

The researchers found out that the dads averagely gained weight from 3.5 to 4.5 pounds, as well as increased their BMI by 2.6 percent. Fathers who did not live with their kids, on the other hand, showed a BMI increase of 2.0 percent.

"Fatherhood can affect the health of young men, above the already known effect of marriage," said Craig Garfield, the lead author of the study, via Washington Post. "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."

The researchers attributed the increase to the changes in the man's lifestyle. "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," said Garfield. "Your family becomes the priority."

The study also showed that men who did not have any children actually lost weight in the 20-year research. Other factors that could affect weight gain, such as race, age, status, income and lifestyle, were accounted for in the study.

Their findings put focus on the ways men, especially new fathers, need to watch over their health. "We now realize the transition to fatherhood is an important developmental life stage for men's health," Garfield said.

Meanwhile, Dr. Eduardo Grunvald, who is not part of the research, but who is the medical director at the University of California, San Diego's Weight Management Program said that the findings were not surprising, but weight gain can make a big difference to men's health as they age.

"I see patients all the time who, when they become fathers, the first thing that happens is their exercise drops off. This study is very interesting because this hasn't been looked at before," he told Today.

The full report, which is a first of its kind about fatherhood, was published in the American Journal of Men's Health.