with my 3rd i didnt gain any fundamental length for 4 weeks (from 31 to 35weeks) and prior to that i was measuring at least 3cms behind
sometimes its how bubs is laying too
my bestie (pregnant at the same time) suddendly measured 3 weeks behind and had a check a week later and right on track it was how bubs was laying and this was in the last few weeks too
hope this helps xoxo
my bubba was measuring small during U/S scans too (i had 6 all up, the last 4 done 19w, 27w, 32w and 36w he was measuring a week behind and small and i had one done a week prior to him being born) and he came out a healthy 3.33kgs(7.5lbs)