Awww Tweeks, it's okay to be nervous/anxious, and I think I myself was the biggest culprit of that pretty much the whole time. But really, the odds are so so so in your favour of everything being just fine, even without prenatals. Like others said, so many women don't even know they're pregnant for quite some time, and often that means not just not taking prenatals, but doing things that might unwittingly harm baby (drinking etc.). Plus women have been having babies for millennia without knowledge about folic acid and yet complications like spin bifida have always been rare. I think it's just an easy thing to take (if you can) that helps avoid that tiny chance.
Please try not to worry too much! I bet you everything will be just fine, and please come and update us!