Ultrasound examination of the pelvic floor during the first stage: a longitudinal cohort study

To investigate changes in hiatal dimensions during the first stage of labour and associations with fetal descent and head position. We conducted a longitudinal, prospective cohort study at the National University Hospital of Iceland, from 2016 to 2018. Nulliparous women with spontaneous onset of labour, a single fetus in cephalic presentation, and gestational age ≥37 were eligible. Fetal descent and position were measured with ultrasound. Three-dimensional volumes were acquired at admission and late in first stage. The transverse hiatal diameter and the levator urethral gap were measured from stored volumes. The study population comprised 78 women. The mean transverse hiatal diameter increased from 39.4 (SD 4.1) mm to 44.3 (SD 5.8) mm (p < 0.019. We found a moderate correlation between the transverse hiatal diameter and fetal station at the last examination; r = 0.44 (p < 0.01) and a weak correlation between the change in transverse hiatal diameter and change in fetal station; r = 0.29 (p = 0.01). Levator urethral gap increased significantly on both the left and right side. Head position was not associated with hiatal measurements after adjusting for fetal station. We found a significant, but only modest increase of the hiatal dimensions during the first stage of labour. The risk of pelvic floor trauma will therefore be low during this stage. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.