They have no idea. My SIL had the growth scan and was told she was having an enormous 4kg+ kid. Her baby got stuck, forceps, he was 3.5kg.

I had a 34 week scan (not for growth, but they measured anyway) Baby was average size, nobody mentioned anything. My belly was fairly small. I had a 4 and a bit kg baby girl, who took her time coming out, but I think she did that on purpose to save me tearing. She wasn't stuck, just slow. I pushed for 2 hours and loved every minut eof the pushing bit. Later, my antenatal midwife told me she was astounded at the size, and that she'd never have picked it, that I "hid it well"

Tell em all to bugger off.

Although... my theory goes that baby size relates to arm strength. I've got sick-arse strong triceps and had a big baby. I need them to hoof her around town