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Abstract: Background:Anal incontinence is an embarrassing condition that is largely underreported. Obstetric anal sphincter injuries are the major etiological factor. Recognition of risk factors may minimize the development of sphincter injuries. The objective of this study was to identify risk factors for sphincter injuries and measure dimensions of mediolateral episiotomies. Methods:Women expecting their first vaginal delivery were invited to participate, and an experienced research fellow performed a perineal and rectal examination and classified tears according to the new international classification. Dimensions of episiotomies were measured and obstetric variables recorded prospectively. Results:Of the 241 women recruited, 59 (25%) sustained sphincter injuries. Univariate analysis revealed that forceps delivery OR 4.03 (1.63–9.92), vacuum extraction OR 2.64 (1.25–5.54), gestation > 40 weeks OR 3.18 (2.35–4.29), and mediolateral episiotomy OR 5.0 (2.64–9.44) were associated with these injuries. In addition, compared with women who had no injuries, sphincter injuries were more common with higher birthweight (3.51 vs 3.17 kg, p < 0.01), larger head circumference (34.3 vs 33.3 cm, p < 0.01), and longer second stage of labor (76 vs 51 min, p < 0.01). Multiple logistic regression revealed higher birthweight and mediolateral episiotomy OR 4.04 (1.71–9.56) as independent risk factors. Episiotomies angled closer to the midline were significantly associated with such injuries (26 vs 37 degrees, p = 0.01). No midwife and only 13 (22%) doctors performed truly mediolateral episiotomies. Conclusions:Mediolateral episiotomy is an independent risk factor for anal sphincter injuries. Although a liberal policy of mediolateral episiotomy does not appear to reduce the risk of such injuries, it may be related to inappropriate technique. A concerted approach to educate trainees in appropriate episiotomy technique and identification of sphincter injuries is imperative to enable reexamination of the true merits or disadvantages of mediolateral episiotomy. (BIRTH 33:2 June 2006)