I measured small on my first pregnancy and it turned out she was just hiding really well as DD went over due by 7days and was born weighting 8lbs
2nd pregnancy i measured small from 33weeks at first I was measuring 2 weeks behind and they werent too bothered but sent me for a scan at 34 weeks when I hadn't grown anymore. Which showed DS was a nearly a week ahead in measurements. Then at 39weeks I was still measuring only 34 weeks so was again still cause for concern. Went for 2nd growth scan and he was still growing an was all in proportion for my actually measurments at term + 6 even though i had 4days till due date. DS eventually arrived the day after my due date and weighted 6lbs 9oz so although he was a tiny little thing I just wasnt carrying all that big on the outside but everything was normal inside.
(may i add i felt like a elephant by term lol)