How much does the levator hiatus have to stretch during childbirth? K Svabı´k, a KL Shek, b HP Dietz b a Department of Obstetrics and Gynecology, First Faculty of Medicine, Charles University in Prague, General University Hospital in Prague, Prague, Czech Republic b Nepean Clinical School, University of Sydney, Nepean Hospital, Penrith, NSW, Australia Correspondence: K Svabı ´k, 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 kamil@svabik.cz Accepted 5 May 2009. Published Online 1 September 2009. 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. Keywords Avulsion, birth trauma, levator ani, levator hiatus, translabial ultrasound. Please cite this paper as: Svabı´k K, Shek K, Dietz H. How much does the levator hiatus have to stretch during childbirth? BJOG 2009;116:1657–1662. Introduction Biometry and function of the levator ani muscle in the human female is likely to be of importance for successful vaginal childbirth. The birth canal is defined not only by the rigid bony pelvis but also by the puborectalis compo- nent of the levator ani muscle. The main purpose of this muscle in the human female is to close off the abdominal cavity caudally while allowing for elimination of faeces and urine as well as for intercourse, requiring limited dimen- sions and distensibility and high resting tone. Vaginal childbirth implies an entirely different set of requirements. The levator hiatus has to distend to a much greater degree, preferably without reaching its elastic limit. The crucial moment in testing muscle properties is the late second stage of labour and crowning of the fetal head, 1 a process that frequently results in substantial trauma. 2–4 There have been attempts at defining the distension required for vagi- nal childbirth with the help of individual data sets acquired with magnetic resonance imaging, 5,6 but from previous work it is clear that hiatal dimensions and distensibility are likely to vary greatly between individuals. 7 Both the unit of the first author 8 as well as others 9 have shown a potential association between hiatal dimensions and delivery out- come. This study was designed to define the degree of stretch/strain required of the levator hiatus (defined by the puborectalis muscle) in childbirth in a large nulliparous cohort. Methods In a retrospective observational study, we analysed the 4D ultrasound data sets of 227 nulliparous women seen at 36– 38 week’s gestation as part of an continuing perinatal inter- ª 2009 The Authors Journal compilation ª RCOG 2009 BJOG An International Journal of Obstetrics and Gynaecology 1657 DOI: 10.1111/j.1471-0528.2009.02321.x www.bjog.org Urogynaecology