low estriol levels and growth scan

In one part of my bloodwork screening, I measured low in estriol. I searched glow and found no one else that has even mentioned this. The doc didn't seem concerned, especially since everything else was normal. Anyhow, they scheduled me for a growth scan tomorrow. I'm excited to see my baby boy again but nervous because I want everything to be okay! Has anyone ever been told they had low estriol?