I tried to explain it by figuring out what was different this time around: baby gender. Two existing kids. Nearing of the magical age 30 that reportedly changes everything about one's metabolism.
But none of those seemed satisfactory: other than fatigue, this pregnancy has not felt any different than my previous ones (except that I can tell I'm carrying lower). Having two kids who play together seems easier than trying to entertain one. My body doesn't feel different now, despite 2 years.
Well, turns out I'm anemic. So since ironing up during the last week, I have felt much, much better.