I measured behind too and wish I had requested more scan. I did have a 32w scan because of the position of the placenta at the 18w scan. I was told I would have a small baby and ended up having an emergency c/sec at 38w due to foetal distress. Bubs was only 4lb2oz and was diagnosed with Intra Uterine Growth Retardation. Next time I will be having fortnightly (possibly weekly) scans from 28w.

Hopefully everything will be OK for you but best that your Dr is overly cautious.