Should I be concerned?

Went and had an ultrasound done yesterday (13w+3D) I am high risk due to previous csection so they had me go to the specialist that does the anatomy scans. He noticed baby has a bit more fluid in the back of the neck, just slightly over normal range. I've been trying not to google things & scare myself but I am a little confused. My dr told me the "normal" range ends at 2.8mm but what I see online many are saying 3.0mm. My baby is measuring between 2.8-3.0mm. He says everything looks great but I am going to get my blood drawn for the harmony test tomorrow and he did bring up other options if necessary. Basic protocol I'm sure but still my mommy senses are kicking in and I'm just trying not to stress. He said the fluid will go down on its own but he does want to keep an eye on it and check again in my scan coming up at the end of next month.