Small baby at 20th week

Hi everyone, I am a 33yr old ftm and I just went for my 20th week detailed scan today and doctor informed us that baby is on the small side. (-1.85) I went for blood test to test for CMV and toxoplasmosis today. They recommended that I went for amnio but we didn't wanna do it because of the risk of miscarriage. We were told to go back again next week to check the blood flow from placenta to the baby. Is this normal? I'm so anxious and worried right now. ?