Does the bump measurement get smaller when.....

[moregranny]

the babys head starts to move down into the pelvis ?

The shape might change but not necessarily the size. Sorry not expert advice but I just seemed to get bigger and bigger and bigger........

My midwife said that she wasn't concerned with the growth/size as the baby had moved further into the pelvis (don't I know it!) so they'd expect it to be measuring a bit less than if it was still 'free'.

I've gone from measuring 38 to 37 in the past 2 weeks, (i'm 40+1) Midwife said baby was on 1/5 palpable and felt very well down so yes you can measure less at the end.