Im sure everything will be fine i was always measuring 4cm under with my first preganncy and had to have regular scans now im 33+5 with my second and ive also got to have a ultrasound next tuesday as im measuring 3cm under what i should be but im not worrying as it was the same with my first imust just carry really well plus my first wasnt a very big baby but he was a good weight he was 6lb 8oz
Your baby may have dropped and is engaged which would cause you to measure smaller. I always measured smaller at the end when baby was engaged. And my babes were 7'6 and 8'5. Pretty nice sized average babies.
Don't worry about it too much, it's common to measure too small or too big, you'll know more after your ultrasound.