I'm 22 weeks pregnant and still haven't had a letter about my 20 week scan. My midwife chased it up at my 16w appointment. I haven't had the consultant appointment that was meant to happen at the same time as the dating scan either but that was my fault as I've had to cancel twice as they keep sending me appointments that clash with the school run. I've got 4 DC's, 2 with SN so lots of appointments already so part of me just thinks I should wait until they send me a letter as I've already got enough to do. However part of me thinks maybe there are some things they check for at 20 weeks that can't be checked later. So does it matter when the 20w scan is as long as it's before I give birth. I see the midwife at 25 weeks so was planning on asking her to chase it up then if I haven't heard anything.
As far as I'm aware the reason it's at 20 weeks is in case they find any serious problems which could result in you choosing not to continue the pregnancy. I've had later growth scans with mine at 28 and 34 weeks, they are still able to get all of the measurements but don't do photos as baby is too big for a good picture.