A.M.
I never did find out what causes them, I dont think they really know. My son was found to have one at about 20 weeks. My doc told me it seems with our increasingly hi tech prenatal screenings, we are seeing more and more of these. It may just be normal, and we are just now seeing them now that everyone gets a level 2 ultrasound. He also assured us (and he is a high risk doctor) it was nothing to worry about on its own. BUT some doctors somewhere think that it is ONE of several markers for down syndrome/chromosomal abnormalities. Your doc/ultrasound tech should have looked for other markers like an echogenic bowl, calcifications on the heart etc. And if you did the AFP, your results from that can help further put your mind at ease, or determine if they need to investigate more, like with an amnio. I know how heartbreaking it can be. We found out at christmas time, when we were finding out the sex--and planning on making a big deal about the reveal of his gender. It was such a scary time for us, but also puts lots in perspective. I wish you peace with this.