20 weeks and fetus is only 4 oz.

GiGi
This is my husband and I first baby and I'm 36 yrs old. I went for my anatomy scan yesterday at 20 weeks but she was so small only 4oz. But so active. I took the 1st trimester blood test and it came back normal. I took the genetic test panorama yesterday. I have to wait a week for the results. Has anyone had this problem? Please pray for me. Thank you