Third and fourth degree tears are unpredictable and often unexpected, but they do tend to occur in relation to some situations more than others. These can include:
Episiotomy
Forceps and ventouse births
Difficult birth of the shoulders
Unusually large baby
Position of the baby
First babies
Fast birth
Position for birth
Swollen perineum
Episiotomy. It is now known that 3rd and 4th degree tears are more likely to occur in association with the use of an
episiotomy, either because the caregiver over estimated the cut or the episiotomy extended. The chances of an episiotomy extending are also increased with the use of forceps or ventouse (vacuum cup). Delivery of the baby is more rapid, not allowing the perineum to stretch slowly. You may wish to read more in
episiotomy and tearing.
Forceps and ventouse birth. The use of forceps can be associated with third and fourth degree tears because the forceps take up more room than the baby's head alone, increasing the chances of injuring the mother. The baby is also brought down the birth canal more rapidly, not giving the perineal tissues adequate time to stretch up slowly.
The ventouse (or vacuum cap) does not take up more room than the baby's head, being similar to an natural vaginal birth, but using it can still stretch up the perineal tissues more rapidly, making them more likely to tear. You may wish to read more in
forceps and ventouse.
Difficult birth of the shoulders. If the shoulders of the baby do not come easily after the birth of the baby's head, the caregiver may need to perform various internal manipulations to rotate the baby's shoulders or deliver their posterior arm first (rather than the anterior shoulder from underneath the pubic bone) to help deliver the baby's shoulders and body. As a way of giving them more room to do this they may perform an episiotomy, which can sometimes extend to a 3rd or 4th degree tear. You may wish to read
difficult birth of the shoulders.