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Objective: Age at menarche and the interval between menarche and age at first vaginal birth have been shown to be associated with reproductive performance; however, their association with maternal birth trauma has not been studied. We aimed to determine whether age at menarche, age at first vaginal birth and their interval are associated independently with levator ani muscle (LAM) avulsion and external anal sphincter (EAS) defect.Methods: This was a retrospective analysis of the ultrasound volume datasets of 466 vaginally parous patients attending one of two tertiary urogynecological units in Australia. All patients had undergone a standardized interview and clinical examination using the pelvic organ prolapse quantification staging system, followed by four-dimensional translabial ultrasound. Tomographic ultrasound imaging was used to evaluate the LAM for avulsion and the EAS for significant defect.Results: Of the 466 women analyzed, LAM avulsion was diagnosed in 121 (26.0%) and significant EAS defect in 55 (11.8%). Logistic regression analysis showed no association between age at menarche and LAM avulsion (P = 0.67). Weak but significant associations were noted between LAM avulsion and age at first vaginal birth (odds ratio (OR), 1.070 (95% CI, 1.03-1.11); P = 0.0007) and between LAM avulsion and menarche-to-first-vaginal-birth interval (OR, 1.064 (95% CI, 1.02-1.11); P = 0.0018). No significant associations were noted between significant EAS defect and any of the evaluated variables (all P ≥ 0.49).Conclusions: Age at menarche is not predictive of maternal birth trauma i.e. LAM avulsion and EAS residual defect. There was a statistically significant association between LAM avulsion and menarche-to-first-vaginal-birth interval; however, this was not stronger than the previously established association between LAM avulsion and age at first vaginal birth, arguing against any distinct effect of prolonged prepregnancy hormonal stimulation on the biomechanical properties of the pelvic floor. © 2021 International Society of Ultrasound in Obstetrics and Gynecology. [ABSTRACT FROM AUTHOR]