The reported incidence of tears involving the anal sphincter occur in 1.7 % (2.9 % in primiparae) in centres where mediolateral episiotomy is practised, compared to 12–19 % (19 % in primiparae) in centres where midline episiotomy is practised [8].

Third- and fourth-degree lacerations can follow any type of vaginal delivery. Many studies have found that episiotomy is the factor with the strongest association with a third- or fourth-degree laceration [9]. Restrictive episiotomy policies appear to have a number of benefits compared to routine episiotomy policies. There is less posterior perineal trauma, suturing, and pain.

Certain other factors have been found to predispose to these lacerations (Table 2.2). A Cochrane Review of 10 trials [10] found that, compared to forceps, the vacuum extractor was associated with less maternal trauma, and therefore, less likely the associate with a third- or fourth-degree laceration. Factors not associated with these lacerations include body mass index (BMI), gestational age, marital status, pre-pregnant weight, weight gain in pregnancy, height, education, time of birth or physical fitness [11].

Taking an overall risk of 1 % of vaginal deliveries, the reported factors are associated with this increased risk of a third-degree tear: birth weight over 4 kg, up to 2 %; persistent occipitoposterior position, up to 3 %; nulliparity, up to 4 %; induction of labour, up to 2 %; epidural analgesia, up to 2 %; second stage longer than 1 h, up to 4 %; shoulder dystocia, up to 4 %; midline episiotomy, up to 3 %; forceps delivery, up to 7 % [12].

A lower risk of third-degree tear is associated with a larger angle of episiotomy. In a prospective case-control study, there was a 50 % relative reduction in risk of sustaining third-degree tear observed for every 6° away from the perineal midline that an episiotomy was cut.

Vaginal delivery has been directly implicated as a major aetiological factor in urinary/faecal incontinence, dyspareunia and uterovaginal prolapse. Although some symptomatic women may present early, the majority tend to present much later, particularly during the perimenopausal years. This delay may be related to embarrassment or to other aggravating factors associated with pelvic floor weakness, such as ageing, oestrogen deficiency and progression of pelvic neuropathy. It is, therefore, crucial to ensure that obstetric anal sphincter injuries are recognized and repaired appropriately. Unfortunately, the majority of secondary perineal and sphincter repairs are the sequelae of unrecognized trauma or inadequate primary repair.

The sphincteric tears are complicated by genital fistulas in 3 % of cases. Maternal injury, as a consequence of failure to recognize and repair anal sphincter tear, is one of the most common causes of complaint and litigation arising in labour ward practice.

The anal incontinence affects the mothers physically and psychologically, and they are too embarrassed to seek help. Only 1/3 of patients with faecal incontinence had ever discussed the problem with a physician.

Table 2.3 reports the symptoms prevalence in third- and fourth-degree tears.

In Table 2.4 are reported the best strategies to prevent a third- or fourth-degree laceration.