When a woman is pregnant, there are dozens of changes to her body and her brain. Fathers, on the other hand, don't go through quite so much. Still, becoming a dad does change men, and those changes are based on science. Evolution prepares both men and women to be good parents. In men who become fathers, there are the usual outward signs of parenthood. Bloodshot eyes from lack of sleep, weight gain or a fitness slump, because there's just no time for the gym, are common, but the brain and hormones of new dads are also changing. Research is mostly limited to heterosexual couples who have biological children, but still, there are changes to men after they become parents.
For one thing, the brain gets larger. MRI studies of new fathers show that just three months after the birth of a child, there is more brain volume. The growth was centered in areas of the brain that process bonding, planning and attachment, which is to be expected. There was some shrinkage, however, which men hate, but hey, it happens. The areas of the brain that process daydreaming shrink in men who have recently fathered a child. Sorry guys, no more time for that fantasy baseball roster. No matter what the impact, having a child is a big deal, even for dads.