Neck fold measurement

I went in for my fetal assestment today. I'm right over 21 weeks. His neck fold measured 6.6 and the ultrasound tech said 6 is normal, so his is a little off. They said it could be a sign of Down syndrome. Everything else was measuring along perfectly though, size and all. My dr told me she's not too worried about it because everything else is measuring normal. It still freaks me out though. ): he wasn't very cooperative as far as getting a picture and some measurements. He had his face pushed up against my uterus most of the time. Have any of y'all experienced this and still had a perfectly healthy baby? I can't help but worry..