Are maternal pelvic floor and bony pelvic dimensions measured by 3D/4D transperineal ultrasound associated with fetal breech presentation?

OBJECTIVES In a cohort of nulliparous women at term, the study aimed to: (1) assess the correlation between subpubic arch angle (SPA) and fetal breech presentation, (2) investigate the relationship between pelvic floor dimensions (at rest, during pelvic floor muscle contraction, and maternal pushing) and breech presentation, and (3) examine the prevalence between levator ani muscle co-activation and breech presentation. MATERIALS AND METHODS In this prospective observational study, 124 nulliparous women at term with a single fetus in either cephalic (n=93) or breech (n=31) presentation were recruited. A transabdominal scan was performed to confirm fetal presentation, followed by 3D/4D transperineal ultrasound (TPUS) to measure SPA, anteroposterior diameter (APD) and transverse diameter (TD) of the levator hiatus, and levator ani muscle hiatal area. TPUS measurements were taken at rest, during maximum pelvic floor muscle contraction, and during maximal maternal pushing. We compared maternal and neonatal characteristics and all TPUS measurements between the two groups. Additionally, we compared the prevalence of LAM co-activation between the two groups. RESULTS Women with a breech fetus had a lower BMI compared to those with a cephalic presentation (25.9±3.9 vs 27.8±5.0 kg/m², P=0.03). They also had a narrower SPA (100±9° vs 109±15°, P<0.001), smaller APD during pushing (52±11 mm vs 57±9 mm, P=0.03), and a smaller increase in APD during pushing, both in absolute terms (2.2±4.8 mm vs 5.4±5.9 mm, P<0.01) and proportionally (3.7±9.3% vs 8.7±9.8%, P=0.02). The prevalence of LAM co-activation was more than two-fold higher in women with a breech presentation (32.2% vs 13.9%, P=0.02). CONCLUSION Maternal pelvic characteristics, especially the subpubic arch angle and the ability to relax the pelvic floor during pushing, may significantly influence the incidence of breech presentation in nulliparous women at term.