Association between maternal and fetal characteristics and levator hiatal dimensions in the second stage of labor

Little is known about the pelvic floor hiatal dimensions in labor, despite this being the soft tissue complex through which the fetus passes in a vaginal delivery. Intrapartum assessment of pelvic floor hiatal dimensions, particularly in the second stage of labor, may improve understanding of the variation in levator hiatal measurements and their change between maternal and fetal characteristics.

To assess the levator hiatal dimensions at rest during the passive second stage of labor using three-dimensional (3D) transperineal ultrasound, participants were consented prospectively, and ultrasound assessment was performed. The study population comprised nulliparous women at term (37-42 gestation) with a live singleton pregnancy.

Participants eligible for participation were recruited for the "SONO-BIRTH" study. Transperineal ultrasound was performed in the second stage of labor. The 3D pelvic floor volumes were uploaded into software to measure the levator hiatal area, levator hiatal circumference, transverse hiatal diameter, and the anteroposterior hiatal distance. Maternal characteristics (age, body mass index, and ethnicity) were collected from antenatal booking forms. Other variables including gestational age and head-perineum distance (HPD) were recorded at the time of the ultrasound assessment, as well as birthweight.

Of 186 participants consecutively recruited, 153 ultrasound volumes were measured. For the maternal characteristic of age, a significant association was observed with increasing maternal age and levator hiatal area (LHa) (β = 0.26 cm², 95% CI 0.08; 0.43, p = 0.004). Differences were also observed in ethnicity, with Asian and other non-European ethnicities associated with a smaller LHa compared to White British (β = -2.92 cm², 95% CI -5.20; -0.0.64, p = 0.012) and (β = -2.70 cm², 95% CI -5.29; -0.11, p = 0.041), respectively. LHa decreased significantly and HPD increased (β = -0.35 cm², 95% CI -0.45; -0.26, p < 0.001). No association was found between hiatal dimensions and body mass index (BMI). In LHa measurements across gestations, a trend was observed toward a larger area with advancing gestational age, although the magnitude of the correlation was small (rho = 0.17, p = 0.031).

We report the relationship between the levator muscle pelvic floor dimensions in the passive second stage of labor and maternal age, BMI, ethnicity, and gestational age. A higher fetal head station is associated with smaller pelvic floor dimensions as is non European ethnicity.