Relationship between the length of the perineum and position of the anus and vaginal delivery in primigravidae.

The aim of the study was to determine perineal length and anal position in primigravidae and to evaluate their effect on vaginal delivery. The distances between the fourchette and each of the center of the anal orifice and the inferior margin of the coccyx were measured in 212 primigravidae with singleton term pregnancies during the first stage of labor. Anal position index was calculated by dividing the first measurement by the second. The mean +/- SD length of perineum was 4.6 +/- 0.9 cm. The mean +/- SD anal position index was 0.49 +/- 0.12. Women with a short perineum (<4 cm) or a small anal position index (<0.42) had significantly higher rates of episiotomy, perineal tears and instrumented delivery. This association was also significant by multiple logistic regression analysis. It was concluded that a short perineum and anterior displacement of the anus were associated with traumatic vaginal delivery in primigravidae.