Superfetation. 10 cases on record. 10 women who, you know in all probability, have said, “Are you SHITTING ME!?!?!?!” to a medical professional.
I think that’s what I was thinking of. For all intents and purposes, you won’t get pregnant while you’re pregnant, but we can’t call it impossible.
“The best form of birth control” is a hard question to answer - there are too many variables for different people.