RESUMEN
OBJECTIVE: To determine the frequency of avulsion of the levator ani muscle in primiparous women according to delivery mode, using introital four-dimensional ultrasonography. METHODS: We performed a prospective observational study at a tertiary obstetric unit. One hundred and eighty primiparous women were included and divided into three groups: normal vaginal delivery without episiotomy, forceps delivery and Cesarean section groups. Between 40 and 120 days after delivery, four-dimensional ultrasonography was performed in order to evaluate the integrity of the levator ani muscle. The operator was blinded to all clinical data and was not aware of delivery mode. The influence of other variables associated with delivery such as birth weight, body mass index, maternal age and use of epidural anesthesia was also studied. RESULTS: Avulsion of the puborectalis component of the levator ani muscle was detected on ultrasonography in 61.7% of women who had undergone a forceps delivery, compared with 13.3% of those who had had a normal vaginal delivery and 0% of those who had had a Cesarean section. Bilateral avulsion was observed in 12/60 (20.0%) of the forceps group and in 2/60 (3.3%) of the normal vaginal delivery group (P < 0.001). Other variables did not seem to influence prevalence. CONCLUSIONS: Forceps delivery is associated with an increased rate of avulsion of the puborectalis component of the levator ani muscle. The effect of forceps use is independent of other delivery-related variables.