IGUR?!?

I had a growth scan done on Christmas eve and got the results in mychart. My baby girl dropped from the 11 percentile to 5.4. They noticed that she was measuring behind at my anatomy scan at 21 weeks and I turned 25 weeks today. At my appt on the 17th my doctor said it could be just due to genetics since we're both very small people. I read that all babies born under 5 lbs 8oz are considered IGUR babies. If that's the case me and all 4 of my siblings would've had IGUR. Can anyone that's had a IGUR diagnosis tell me your experiences or if you know anything about IGUR? I'm really worried and I won't be able to even call my doctor's office until Monday.