Bladder neck mobility is a heritable trait H.P. Dietz, a N.K. Hansell, b M.E. Grace, b A.M. Eldridge, b B. Clarke, c N.G. Martin b Objective Congenital connective tissue dysfunction may partly be responsible for female pelvic organ prolapse and urinary incontinence. We undertook a heritability study to determine whether mobility of the bladder neck, one of the main determinants of stress urinary incontinence, is genetically influenced. Design Heritability study using a twin model and structural equation modelling. Setting Queensland Institute of Medical Research, Brisbane, Australia. Population One hundred and seventy-eight nulliparous Caucasian female twins and their sisters (46 monozygotic pairs, 24 dizygotic pairs and 38 sisters) aged 18–24 years. Methods We performed translabial ultrasound, supine and after bladder emptying, for pelvic organ mobility. Urethral rotation and bladder neck descent were calculated using the best of three effective Valsalva manoeuvres. Main outcome measures Bladder and urethral mobility on Valsalva assessed by urethral rotation, vertical and oblique bladder neck descent. Results Genetic modelling indicated that additive genes accounted for up to 59% of the variance for bladder neck descent. All remaining variance appeared due to environmental influences unique to the individual, including measurement error. Conclusion A significant genetic contribution to the phenotype of bladder neck mobility appears likely. INTRODUCTION Urinary incontinence and prolapse of the pelvic organs are common complaints in women, leading to significant morbidity from the fourth decade of life onwards. 1 Directly or indirectly, they account for 10–30% of the workload of the gynaecological surgeon, and the lifetime risk of having to undergo anti-incontinence or prolapse surgery has been estimated at 11%. 2 The main determinant of stress incontinence seems to be bladder neck descent (BND) on Valsalva, 3 and the rel- evance of organ mobility for the condition of pelvic organ prolapse is self-evident. It is likely that childbirth is the major environmental factor in the development of prolapse and stress incontinence, 4,5 and parturition seems to have a deleterious effect on pelvic organ support. 6 However, ge- netic factors probably also play a significant role, and the wide variation in this phenotype in young women suggests a genetic contribution. 7 Certain genetically determined con- nective tissue abnormalities (such as Marfan’s and Ehlers Danlos syndromes) cause clinical conditions which are associated with skin, fascial and ligamentous relaxation as well as incontinence and prolapse, 8,9 and a family history of prolapse is a recognised risk factor. 10,11 Consequently, it seems reasonable to suspect a genetic contribution to the phenotype of bladder and urethral mobility. In order to determine the need for population-genetic or molecular-genetic approaches, we undertook a study to define whether bladder and urethral mobility on Valsalva manoeuvre are a heritable trait. METHODS Caucasian females aged 18 to 24 years had initially been approached through mailouts to secondary schools in the Brisbane region to participate in genetic studies of mela- nocytic naevi and cognitive function. Zygosity was deter- mined using a commercial kit and cross checked with blood group and other phenotypic data, giving an overall proba- bility of correct assignment of greater than 99.99%. All women gave informed written consent. They re- ceived a shopping voucher worth A$100 for their partici- pation. Ethics Committee approval had been obtained from the local Ethics Committee (QIMR P434 (H0202-01-004)). In a detailed interview, we questioned bladder (stress and urge incontinence, frequency, nocturia, symptoms of void- ing dysfunction and urinary tract infections) and bowel symptoms (straining at stool and chronic constipation), as well as a history of complaints associated with connective tissue dysfunction (dislocations, epistaxis and herniae), a history of bedwetting beyond school age and knowledge BJOG: an International Journal of Obstetrics and Gynaecology March 2005, Vol. 112, pp. 334–339 D RCOG 2004 BJOG: an International Journal of Obstetrics and Gynaecology www.blackwellpublishing.com/bjog a University of Sydney, Australia b Queensland Institute of Medical Research, Brisbane, Australia c Royal Women’s Hospital, Brisbane, Australia Correspondence: Mr H. P. Dietz, University of Sydney, Western Clinical School, Nepean Hospital, PO Box 63, Level 5 South Block, PENRITH NSW 2751, Australia. DOI:10.1111/j.1471-0528.2004.00428.x