Baby’s head circumference is small at week 28 growth scan

I went for a growth scan at week 28 as I was found out to be of high risk preeclampsia at my 2nd trimester detailed scan. During that growth scan, my baby’s head circumference was smaller than his gestational age. At week 31 + 3 days, did another scan and baby’s HC is at week 30. My gynae suggested another growth scan at week 33 and to check brain’s structure but all the detailed scan before this showed no abnormalities to the brain structure. Should I be worried? Can the brain structure change at such a late stage of pregnancy? Thanks!