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Objectives -To determine (i) risk factors in the development of third degree obstetric tears and (ii) the success of primary sphincter repair.Design -(i) Retrospective analysis of obstetric variables in 50 women who had sustained a third degree tear, compared with the remaining 8,553 vaginal deliveries during the same period. (ii) Women who had sustained a third degree tear and had primary sphincter repair and control subjects were interviewed and investigated with anal endosonography, anal manometry, and pudendal nerve terminal motor latency measurements.Setting -Antenatal clinic in teaching hospital in inner London.Subjects -(i) All women (n=8,603) who delivered vaginally over a 31 month period. (ii) 34 women who sustained a third degree tear and 88 matched controls.Main outcome measures -Obstetric risk factors, defaecatory symptoms, sonographic sphincter defects, and pudendal nerve damage.Results -(i) Factors significantly associated with development of a third degree tear were: forceps delivery (50% v 7% in controls; P=0.00001), primiparous delivery (85% v 43%; P=0.00001), birth weight >4 kg (P=0.00002), and occipitoposterior position at delivery (P=0.003). No third degree tear occurred during 351 vacuum extractions. Eleven of 25 (44%) women who were delivered without instruments and had a third degree tear did so despite a posterolateral episiotomy. (ii) Anal incontinence or faecal urgency was present in 16 women with tears and 11 controls (47% v 13%; P=0.00001). Sonographic sphincter defects were identified in 29 with tears and 29 controls (85% v 33%; P=0.00001). Every symptomatic patient had persistent combined internal and external sphincter defects, and these were associated with significantly lower anal pressures. Pudendal nerve terminal motor latency measurements were not significantly different.Conclusions -Vacuum extraction is associated with fewer third degree tears than forceps delivery. An episiotomy does not always prevent a third degree tear. Primary repair is inadequate in most women who sustain third degree tears, most having residual sphincter defects and about half experiencing anal incontinence, which is caused by persistent mechanical sphincter disruption rather than pudendal nerve damage. Attention should be directed towards preventive obstetric practice and surgical techniques of repair.