Measuring small at anatomy scan?

Caitlin • EDD 5/4/19 with twins! 🌈🌈
I had my anatomy scan today at 19w6d and baby is measuring 4 days behind. She is 10oz. Anyone else have a baby that was measuring behind instead of ahead? 
I had a dating ultrasound at 6w, another at 12w and one at 15w. She has always measured on point.