This one is quite obvious since 100% of all kids who are too young to know about sex would believe, that babies would be conceived anally, and periods were just random bleeding, like piles. Gross but hilarious.
A childhood belief about getting pregnant

others believed things like this
- When I was young I had a plan to shoot big piles of paper up to the sun so it would never burn out.— Waterbottle
- On TV game sows when they said the winners were chosen "at random" I thought random was a town.Anon
- i used to believe when iwas young that during periods, one bleeds through thier breastsAnon
- I used to believed that the clouds in the sky were floating piles of mash potatoes .— Funny Guy
- I used to think that kidneys were where babies ('kids') came from. — Ri
- when i was young i used to think babies just poped out like a little pistle and the doctor caught itAnon