Objective To assess the relationship between the subpubic arch angle, anal sphincter and perineal trauma, and anal incontinence after childbirth in nulliparous women.
Design Prospective cohort study.
Setting University teaching hospital.
Sample One hundred thirty-four low risk nulliparous women.
Methods Measurements of the bony pelvic outlet were made to calculate the subpubic arch angle. Endosonographic assessment of the perineum and anal sphincter was made before and after delivery. A specific bowel function questionnaire was completed before and after delivery.
Main outcome measures Perineal and anal sphincter trauma, and deteriorating anal continence after delivery.
Results Thirty-two women with a subpubic arch angle of less than 90° had significantly prolonged first and second stages of labour when compared with 102 women whose subpubic arch was wider. Following delivery, anal continence deteriorated in more women with a narrow subpubic angle (69%vs 21%, P < 0.001) but this was unrelated to the incidence of anal sphincter and perineal trauma.
Conclusions A narrow subpubic arch is strongly associated with prolonged labour and postpartum anal incontinence in nulliparous women. However, perineal and anal sphincter trauma, assessed by ultrasound, does not account for the higher rate of postpartum anal incontinence in women with a narrow subpubic arch angle.