Does the fetal head station and position affect the pelvic floor muscles in labour? A prospective study using 3...

The passage of the fetus through the birth canal, stretches the soft tissues of the pelvic floor, in particular the levator ani muscle. Excessive distension of the levator ani muscle (LAM) hiatus and LAM avulsions are associated with pelvic organ prolapse. Our aim was to evaluate the impact of the fetal head position and station on the LAM.

A prospective cross-sectional observational study of women undergoing their first vaginal birth. Women were examined vaginally by a doctor or midwife to assess the fetal head station in relation to the ischial spines. Three dimensional transperineal ultrasound (3D TPUS) was performed on these women in the second stage of labour when they had a vaginal examination. The 3D TPUS was done to identify LAM avulsion and measure the anteroposterior (AP) diameter and the hiatal area. In addition, transabdominal ultrasound (TAUS) was used to determine the fetal head position. A Kruskal-Wallis test was performed to compare non-parametric variables.

274 women were invited and 264 (95 %) agreed to participate. 52 women had a TPUS performed during the second stage of labour. The fetal head position was occiput anterior (OA) 32 (62 %), occiput posterior (OP) 9 (17 %), and occiput transverse (OT) 11 (21 %). There was a significant increase in the AP diameter and hiatal area as the fetal head descended from -1 to +2. (AP diameter: 6.1 vs 8.1 cm, p = 0.002; hiatal area: 16.3 vs 30.3 cm², p = 0.01). The fetal head position did not affect the AP diameter or hiatal area measurements. No LAM avulsions were diagnosed in the second stage of labour before birth. No LAM avulsions were found following caesarean section (n = 7). Women who gave birth vaginally were invited to have a repeat scan after three months, and 35/45 (78 %) came for follow-up. LAM avulsions were diagnosed three months postpartum in 10/35 (29 %) women following their vaginal birth.

This is the first study to evaluate how the fetal head station and position affect the LAM after active second stage of labour. There is a 25 % increase in AP diameter and a doubling of the hiatal area as the head descends from station -1 to +2. LAM avulsions are known to occur following a vaginal birth, and this study demonstrates that LAM avulsions do not occur until the birth of the head. It also highlights that despite pushing in the active second stage of labour, an unsuccessful vaginal delivery followed by CS is not associated with a LAM avulsion. This information will be useful to counsel women regarding mode of delivery.