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


K Svabík, Department of Obstetrics and Gynecology, First Faculty of Medicine, Charles University in Prague, General University Hospital in Prague, Apolinarska 18, Prague 2, 128 00, Czech Republic. Email


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.