Anogenital distance in adult women: examining a new biomarker in gynaecology

Anogenital distance (AGD) is proposed as a marker of the prenatal hormonal environment and reportedly differs in women with endometriosis, polycystic ovary syndrome and impaired fertility. Before clinical use, its normal range, determinants and measurement reproducibility must be established. We asked what normal AGD values are in premenopausal adult women, which participant characteristics are associated with AGD, and how reproducible AGD measurement is within and between observers. Cross-sectional study at a tertiary public gynaecology service, the Royal Women’s Hospital, Melbourne, Australia. Women aged 18 to 50 undergoing benign gynaecological surgery under general anaesthesia were recruited between March 2021 and September 2022; 307 of 324 were eligible and analysed. Demographic, reproductive, contraceptive and smoking data were collected by purpose-designed pre-operative questionnaire; BMI came from the medical record. AGD-AC (anus to clitoris) and AGD-AF (anus to posterior fourchette) were measured with disposable millimetre rulers in lithotomy under anaesthesia at two time points by one or two clinicians. Associations were assessed using linear regression and one-way ANOVA, with multivariable models for each measure. Observer agreement was quantified as mean absolute and relative differences between paired measurements. Analyses were complete case and two-sided, with p < 0.05 significant. Both measures were normally distributed. Mean AGD-AF was 35.9 mm (SD 8.8, 95% CI 34.9 to 36.9) and mean AGD-AC 121.0 mm (SD 19.4, 95% CI 118.9 to 123.2). BMI was associated with both measures and remained so after adjustment; parity was associated with AGD-AC in the multivariable model. Neither measure differed by ethnicity, smoking status or contraceptive use. Mean intra- and inter-observer differences were 4.5 mm (3.7%) and 9.5 mm (8.0%) for AGD-AC, and 3.2 mm (9.1%) and 6.4 mm (17.9%) for AGD-AF. AGD-AC was the more reproducible measure but was associated with BMI and, after adjustment, parity. AGD-AF was associated with BMI alone but showed markedly greater relative measurement error. For both, inter-observer error exceeded half the differences previous studies reported in endometriosis. These findings constrain the use of AGD as a clinical biomarker; studies using AGD should adjust for BMI and parity and report measurement reproducibility. Australian New Zealand Clinical Trials Registry, ACTRN12621000682842. Registered 4 June 2021. Retrospectively registered.