It's a tricky one because midwives know what they're doing, they've done it a million times, but we know our bodies and our cycles - not to mention when we dtd!!
At my 8 week scan I was spot on, but at my 10 week scan they suggested I might end up getting put back up to a week at my 12 week scan. Now I know for a fact this isn't physically possible, as I obviously know when we last dtd etc, so it would make it physically impossible to have conceived a week later!!
DH joked that it wouldn't be his if we got put back a week, so even he knows when we last dtd!!
Secondly, it would also have meant that I got a bfp at 5dpo, which we all know would never happen.
So in a way, measurements are good, but at the same time we are good judges (especially those who have being ttc for a while and know their cycles) of our own bodies.
The problem for me is, I only have a one week window for my eligibility for maternity pay, as I started a new job after my bfp. If they put me back a week I'll not be eligible