OBJECTIVE: This study was designed to define the degree of stretch/strain required of the levator hiatus in childbirth. There have been attempts at defining the distension required for vaginal childbirth with the help of individual data sets, but from previous work it is clear that hiatal dimensions and distensibility are likely to vary greatly between individuals. DESIGN: Retrospective observational study. SETTING: Nepean Hospital, University of Sydney. POPULATION: Nulliparous women at 36-38 week's gestation. METHODS: The ultrasound data sets of 227 nulliparous women examined at 36-38 week's gestation were investigated using post-processing software. Minimal hiatal diameters, subpubic arch, circumference and area were measured at rest, on Valsalva and pelvic floor muscle contraction. To estimate required hiatal distension at vaginal birth we used neonatal biometric data obtained in a Caucasian population. The muscle 'strain' or 'stretch ratio' required to allow delivery of a Caucasian baby of average size was calculated from dimensions at rest and on maximal Valsalva. MAIN OUTCOME MEASURES: Degree of stretch/strain required of the levator hiatus in childbirth. Results The mean strain (stretch ratio) required for vaginal delivery was calculated as 1.47 (range 0.62-2.76; SD 0.39) from resting length, and 1.07 (range 0.25-2.45; SD 0.44) when calculated from dimensions at maximal Valsalva. This implies that, from dimensions at maximal Valsalva, some women will have to distend only 25%, others by 245%. CONCLUSIONS: We have obtained normative data for the required distension of the levator hiatus in a largely Caucasian population.

Department of Obstetrics and Gynecology 1st Faculty of Medicine Charles University Prague General University Hospital Prague Prague Czech Republic

References provided by Crossref.org

Persistent occiput posterior position and stress distribution in levator ani muscle during vaginal delivery computed by a finite element model

Fetal head size and effect of manual perineal protection

Finite element model focused on stress distribution in the levator ani muscle during vaginal delivery

The role of thumb and index finger placement in manual perineal protection

Modeling manual perineal protection during vaginal delivery