I used to believe as a kid that when your Mum got pregnant, the baby grew in her body standing upright. eg: The baby's legs grew in the Mother's legs and it's arms grew in the mother's Arms.
What makes it worse, I used to think that the baby could see out of the mother's eyes and that's how it knew where it's home and family was.... lol
A childhood belief about reproduction

others believed things like this
- I used to think that up in outer space the sun was a gigantic baby that never grew up like in Teletubbies.— WeirdButProud
- I thought a Monkey was just a baby Gorilla and when they grew up their Tails fell off!— Brainhead
- I believed that women who wanted babies ate a seed, which then grew into their baby. — World's Worst Scientist
- I believed (thanks to my mum) that babies grew on "baby trees" and simply 'fell off' when they were ripe!!!— Stace
- When I was little I believed that the mother vomited the baby. Anon
- I used to beleive that all pains were headaches. So for example, I had headaches in my stomach, legs and arms.— James