Aih, I understand la your worry, a few of us mums have gone through the same thing masa pregnant! Honestly, head circumference yang slightly besar compared to dating doesn't always mean something is wrong, tapi this really needs proper ultrasound follow-up with your doctor or gynae to rule out anything serious like polyhydramnios or other conditions. My friend's scan showed similar reading, turns out baby dia memang big head je genetically, everything fine la in the end, but the key is kena monitor properly with your specialist, don't stress alone k!