Lacerations are classed by their severity. Second degree lacerations involve muscle tissue. An episiotomy is equal to a second degree laceration in terms of which tissues it involves. Third degree lacerations involve the anal sphincter muscle. Fourth degree lacerations involve the rectal mucous membranes.
Most spontaneous tears are only first or second degree.
Third and fourth degree tears rarely occur spontaneously if the woman has not had an episiotomy. It is not unusual, however, for episiotomies to extend into third or fourth degree tears. So in reality, the risk of having a serious tear is much greater with an episiotomy than without one. Episiotomies are easier to repair than tears and cause less pain during healing. Most tears are small and straight.