Growth scans

[1Pinkfluffyelephant]

Does anyone have experience with growth scans in late pregnancy - post 37 weeks. If the head is engaged as confirmed by midwife, how do they measure the head circumference and bpd to create an accurate weight estimation?

I had one last week (I was 36 weeks exactly) and head was engaged, sonographer struggled to get head circumference as she was so low but managed to do so in the end. Got another scan in 2 weeks if she isn't here by then! x