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 distanc es between the fourchette and each of the center of the anal orifice and th e inferior margin of the coccyx were measured in 212 primigravidae with sin gleton term pregnancies during the first stage of labor. Anal position inde x 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 an al position index (<0.42) had significantly higher rates of episiotomy, per ineal tears and instrumented delivery. This association was also significan t by multiple logistic regression analysis. It was concluded that a short p erineum and anterior displacement of the anus were associated with traumati c vaginal delivery in primigravidae.