From my perspective, the answer to the title question is clear-cut: the bad advisor. But people like to blame the baby.
“Blame” is maybe an overly hostile word. It almost always comes out as encouragement, as in, “You shouldn’t worry that it’s going to take you longer than average. You had a baby!” And I certainly don’t want to downplay the exhaustion of pregnancy and the even greater exhaustion of recovering from birth while caring for a newborn. That does cut down on one’s productivity for a while.
But parenting also brought with it a surprising productivity boost: time management. I am much better now at Getting Stuff Done while in the office. My work days are much more goal-oriented. I don’t allow myself to procrastinate anymore, because I can’t afford the time, and that’s made a huge difference. I’d estimate my net productivity was really only down for about four months, plus some time lost here and there from low energy levels during pregnancy.
The bad advisor, on the other hand? She cost me two-and-a-half years. Leaving meant starting an entirely new project for my dissertation. One could argue that I gained some skills during those two-and-a-half years, so they weren’t a total wash, but one could also argue I lost even more time combating the subsequent lack of motivation.
So the baby comment makes me feel sort of weird, and the bad ex-advisor still makes me rage-y and bitter sometimes.