I only had it once out of my 4pgs and that was with my 2nd, but I was more concerned about her still being ok because 4days before my appointment I had very significant bleed and was terrified I'd lost her so in that respect the timing was excellent. I think there is too much room for error due to the operator and even if your dates are the slightest bit out, it will affect the measurements too.

If you know that the outcome of the scan wont affect your decision to continue the pg, then dont have it kwim?