How much does the levator hiatus have to stretch during childbirth?

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.