I used to think that if you had sex while you were pregnant that you would have twins, triplets, etc...based on the amount of times you had sex.
A childhood belief about reproduction

others believed things like this
- That you could give birth to what ever you chose (i.e a girl, a boy, twins, triplets)— Krista Durling
- 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