I used to believe that you have conjoined twins by having sex during the pregnancy, making the two fetuses attch to each other.
A childhood belief about reproduction

others believed things like this
- i always thought twins had the same name as each other— Elle
- i believed if you had sex twice in a row you would have twins— dona
- I used to believe that twins were always identical.Anon
- My friend used to think that having a threesome would mean you would have twins.— Yank
- i used to believe i would 'catch up' to my big brother one day and we'd be twins.— lil' sis
- I used to think that a company had boats all over the world making waves, and airplanes making cloudsAnon