2D USS of the pelvic floor in the 3rd trimester versus mode of delivery.

Objective: Biomechanics of pelvic floor muscles is becoming an important field of research in understanding a complex process of labour. Current evidence indicates that the levator ani stretches to allow passage of the fetus through the birth canal. We sought to identify properties of the functional female pelvic floor during pregnancy that could help to predict the mode of delivery, especially in women who underwent caesarean section in the past.
Methods: A single centre prospective cohort study of pregnant women in the 3rd trimester attending antenatal clinic. Participants were categorised into three groups: 1. nulliparous, 2. previous one vaginal delivery and 3. previous one caesarean section who chose a trial of labour in their current pregnancy. All women underwent ultrasonographic evaluation of their pelvic floor muscles. Distensibility and the mode of delivery were assessed in participants' current pregnancies. The primary outcome was the mode of delivery. ANOVA test was used for statistical analysis.
Results: 133 women were included: 55 nulliparous, 52 primiparous (previous one vaginal birth), and 26 primiparous (previous one LSCS and planned for vaginal delivery after caesarean (VBAC)). Participants were scanned at an average gestation of 32 weeks. The inter-observer reliability test for puborectalis distensibility was good (kappa 0.75). Overall, women who delivered vaginally had a more distensible puborectalis muscle than those who delivered by emergency LSCS regardless of their previous type of delivery. Women who underwent a successful VBAC had less distensible pelvic floor muscles compared to their nulliparous and primiparous counterparts (who delivered vaginally in the past).
Conclusion: These results support our basic conceptual theory of puborectalis muscle distensibility influencing the mode of delivery and endorse application of dynamic pelvic floor ultrasound in future studies. We identified measurable differences in the functional female pelvic anatomy aiding a better understanding of physiology of the labour. There are trends towards lesser distensibility in women who delivered by LSCS.
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