Mum had 3 - one a week late, normal delivery about 14 hours; one 2 wks early, normal but bad delivery, mega blood transfusion; me 2 wks late, vaginal breech birth, NO PAIN RELIEF (anaethetist had been up all night, was a bit tired so decided was not returning to hosp!!!).
Me - 1st, 40+7, 15hrs, normal delivery, PPH; 2nd, 39+4, 22hrs, normal delivery; 3rd, 38 (induced), normal delivery.
DSis - 36hrs, forceps, lots of damage (to her, not baby - he was 9lb something bruiser - literally)
I think it's a myth. Unless your mum has a particularly small pelvis and you inherit that from her.