When do I start prenatal vitamins? Before stopping BC or after?

I want to get pregnant in a few months so I started taking prenatal vitamins but I'm continuing my BC pill until I begin ttc. But now I'm wondering if I need to take the prenatals before stopping the pill or after stopping the pill? Because what if I get pregnant right away after stopping the pill? And then I haven't had enough prenatals. Lol help a girl out!