Fluid behind the neck? Help!

Lydia
Today I had my first scan.. Baby was amazingly beautiful and active. However, the fluid on the back of baby's neck should be approx 3.5mm or under but my baby had 3.9-4.0 I am now being sent for different tests to do with Down syndrome and also tests to see if baby's heart is ok. They said it's just to be sure baby is ok but I'm terrified. 
Someone out there please tell me they've had this and it's been ok?