I found it was the size of the head (36cm) that did the damage for me, I had 2x 8lb 10oz bubs and a 9lb 4oz bub and had 2nd degree tearing with each birth, how ever there was no trouble with them moving through the birth canal. I tried to manage the delivery so as not to tear but to no avail. Yet you find that other ladies birth smaller bubs and have lots of trouble. IMO it has a lot to do with the makeup of the mum and how the labour is managed (also I think we need to be looking at the size of you partners head and asking how big he was at birth before you fall in love and make babies)