My DS's head was measuring at 40 weeks when I had my 36 week check up so they sent me off for a growth scan. They told me he was a big baby. But, when he was born he was 3.67kg (just over 8 pounds) and his head circumference was 36.5cm (I was told that was on the larger side, but normal). I think some babies just have big heads, even now his head is a little bit bigger (55th percentile, when the rest of him is 50th percentile).