Experience with Neural Tube Defect or AFP Test?

I'm 21 weeks pregnant and my doctor called me today with the results of my AFP (alpha fetaprotein) test and told me that they indicated my baby is positive for a neural tube defect (my AFP level was elevated). She also told me that there are alot of false positives with this test and the rate of false positives is 1 in 75. I have no history of neural tube defect in my family, I am 20 years old, my husband is 22 and has no history of this in his family, I was taking prenatal vitamins with folic acid for three months before I conceived, and my doctor did not notice any abnormalities in the baby at my 19 week scan. Does anyone have a similar experience or any information on neural tube defects or false positives with an AFP blood test in the second trimester that they can share? I'm really worried and I cannot get in to have a high definition ultrasound until next Wednesday. Any information would be highly appreciated!