I was told that at the 20 week scan that they dont usually get concerned unless the baby measures more than i think she said 2 weeks either way. My DS was a week behind in measurements, so was measuring only 18 weeks when i was 19 weeks at my morph scan which id say was before a growth spurt as he soon caught up and was a very healthy 8lbs at birth.

I'd say he recently had a growth spurt

FWIW, my sister was told her DD was 9lbs at 37 weeks, she went into spontaneous labour at 38 weeks and her DD was 7lbs 13oz.