my scans were accurate for all my kids
they told me my 3rd would be 4kg plus if i went full term- however my body told me he was ready at 37+5 and he was delivered naturally /no drugs/ no probs and weight 3.6kg- so he would possibly be over 4kg if i went full term. They tried to scare me with the size as well but it never got a chance to be an issue- i wasnt worried- i knew my body would only bake a baby big enough for my body to birth!
btw its the size of the head that is the ouch rather than the baby's weight lol
good luck