DD fundal height was 31 weeks at 36 weeks and didnt increase because they drop lower into your pelvis after this date. perhaps this is what is happening. Also dd was laying head down but transverse which prevented my belly from sittingout a lot.

Also at 36 weeks they did another scan as i was only measuring 31 weeks and they guessed dd was
5 lb 14oz, when she was born at term she was 5lb 13oz, so obviously scanmeasurements were incorrect as i doubt she did grow in the last 4 weeks!