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Because obstetric injury to the anal sphincters may be occult, and because the mechanism of injury differs between first and subsequent deliveries, we prospectively assessed the effects of first and second vaginal deliveries on anal physiology and continence.We undertook a prospective observational study of 59 previously nulliparous women through two successive vaginal deliveries by means of a bowel-function questionnaire, and an anorectal-physiology assessment, both antepartum and 6-12 weeks post partum.13 (22%) women reported altered faecal continence after their first vaginal delivery: eight had persistent symptoms during their second pregnancy, of whom seven deteriorated after the second delivery; five regained continence before their second pregnancy, but two became incontinent again after the second delivery. Five women developed incontinence for the first time after their second vaginal delivery, of whom three had occult primiparous sphincter injury. 20 (34%) women, seven of whom had no symptoms, had anal-sphincter injury as a result of their first delivery, but only two new injuries occurred after the second vaginal delivery (p=0.013). Although pudendal neuropathy was no more common after the second than after the first vaginal delivery (15 vs 19%, p=0.8), pudendal-nerve latency was longer after the second delivery (p=0.02).Primiparous women with persistent symptoms of altered faecal continence experience deterioration after a second vaginal delivery. Women with transient faecal incontinence or occult anal-sphincter injury after their first vaginal delivery are at high risk of faecal incontinence after a second vaginal delivery. The risk of mechanical anal sphincter injury is greatest after the first delivery.