My heart.
When we found out we were expecting our first child nearly nine years ago (!) I was unprepared to the point of near-lunacy. I ended my club-girl partying days approximately one second before we decided to try for a baby, lived in a one-bedroom fourth-story walkup with holes in the floor and an oven that wanted to kill us, and had exactly zero friends with children (in addition to having no siblings or immediate, close-by family members with kids).
The second time around, I was better prepared. I knew to advocate for myself. I knew that PPD was a real thing, and that I was likely to have it. I knew what I needed (diapers) and what I did not (10,000 newborn-sized onesies).