My mom used to have a friend who was in a large Catholic family with 12 siblings. They ate a lot of beans. Thus, Mom made the connection that beans made you pregnant.
A childhood belief about getting pregnant

others believed things like this
- That baked beans were made of pasta! Spaghetti is?!Anon
- I used to think people were called "human beans".— Dave
- I used to call the rays of light from the sun "sun BEANS". That's what it sounded like to me.Anon
- My mum used to tell me that baked beans were just peas covered in tomato sauce.— munky
- Until just a few seconds ago, I thought gherkins were beans or something weird like natto...Anon
- I used to believe 'three bean salad' was an extremely small entree because it contained only three beans.— queeneve