For ages my FM matched my weeks exactly.. but towards the end they went a bit off- i went into labour at 37 +3 weeks, and my last FM was about 35cm i think.. my boy was
6lb3, so not too big, but good for being a bit early- he never had to go in a crib or anything.

I think too that it depends on wether bubs is engaged aswell- DS was engaged for a few weeks before he arrived (not fully- but always 3/5 or 4/5) at least thats what i told myself whenever the measurement was out so i didnt freak about him being too small!!