Did some pubmed searches, found some studies:

The perineal body length and injury at delivery.

Risk factors for perineal injury during delivery.

Is primiparity, the only risk factor for type 3 and 4 perineal injury, during delivery?

Basically i found that i have an extremely short perineum. Long is over 6cm, average is 3-6cm, short is less than 3cm. The studies which looked at risk factors concluded forceps, posterior presentation and being a first time mum posed the most risk. In the perineal length study, perineal length was found to have no bearing on damage whatsoever.

Bx