A lot of men gain weight when women are pregnant in the second/third trimester because the woman is upping her caloric intake = larger meals = husband is probably eating more as well. Then there's stuff like "midnight snacks" (she gets up and eats something, and even if it's healthy he might also get up and eat something as well).  Then when the baby is very little they're short on sleep, which means dad is probably not going to the gym as often. He might decide things are stressful and eat a crappy lunch or maybe he's drinking more beer than usual, not because he's a drunk, but because he feels he deserves to "unwind" as well. And while mom went for a walk with baby for his/her afternoon nap, dad didn't do that-he rushed home after work. He also feels pressure to work harder because maybe mom's income is on hiatus for a while or because he wants to provide as well as he possibly can. He's sitting more. He's worn out. Of course, awareness around all of this is what keeps people from getting fat. Keep healthy food around, do a bit of meal prep if you can, work out in the morning, and once your kid is around six months of age, play with them as much as possible. A two year old will run around for HOURS and be delighted if dad is actively chasing them etc. at the park instead of lazing on the bench like everyone else.