For women in whom it's their first labour (primigravida) there will usually be some degree of tearing. Some first time mums are lucky and have no tear. For multipara (second time mums and above) tearing is usually minimal and some do not tear at all. This is because the soft tissue resistance is less in women have delivered before and secondly the perineum tends to stretch out better in these women as well.
Likelihood of tearing is related to a few factors. These include size of baby, need for assisted delivery with vacuum or forceps, as well as the expulsive forces at the point of delivery itself. Generally slow control pushing right at the point of crowning is preferable as opposed to a sudden strong push.
However during labour it's often difficult for mum to control how she pushes even with guidance. Sometimes the urge to push can be very strong and labour can be precipitate. That means the baby comes out very rapidly and suddenly. The role of the birth attendant, whether it be a midwife or obstetrician, is important in guarding the perineum to control the degree of tearing.
