There are floods of ads on TV all the time about taking prenatal vitamins before becoming pregnant. But if you're like me, and weren't trying, chances are very high that you haven't been. And this is OK. When I found out at the doctors at around 6 weeks, the doctor wrote down a vitamin for me to grab on the way out, to start taking once a day, as it's really important especially in the first 12 weeks to be giving your baby all of these vital nutrients.