I thought that girls gave birth by having their bellies burst open kind of like in Alien but bigger. The weird thing is - I had never watched Alien when I thought this as a kid. I think it might have been because I thought my mom said my old dog had a scar on her belly from giving birth, but when I asked her a few months ago she said she never said that.
I thought girls bled from their nipples when they were on their period - I never knew how the vagina worked. (And I didn't really know what a "period" was either, maybe a general concept but ... no idea how I even thought of this).