I guess it's inevitable. Hit kid #5 (pregnancy #6), and someone will ask you "Don't you know what causes that?"
Given the surface stupidity of the question, I am forced to assume it is asked out of honest ignorance. And so, I have decided to answer the question, with the assistance of drawings of questionable quality:
Now, I'll admit that this is a grossly oversimplified answer, but it covers the basics. (And yes, I realize full well that that's a vulva up there, not a vagina, but you try to draw something hidden inside the body from the outside & tell me how it goes.)
If ever you are prompted to ask a pregnant woman if she knows what causes her condition, refer back to this drawing to remind yourself.