High AFP Levels...

I am a little over 16w, and received a call yesterday that I screened positive for the raised AFP levels that could indicate my baby as having spina bifida. The nurse on the phone said not to freak out, because it was just a screening, not a diagnostic test. But, naturally, I just can't stop stressing. I did some research, and my prenatal contains adequate amounts of folate. So I want to think that perhaps they just misjudged how far along I am. I saw that the initial numbers indicated it was unlikely, but also that after false positives, every 1 in 2-500 babies ended up having it. If anyone else has been through this, I could use some advice. I'm stressing so badly about my baby.