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33 Prevalence of functional defecation disorders 1 INTRODUCTION The umbrella term functional defecation disorders (FDDs) is used to describe two distinct diagnoses; functional constipation (FC) and functional nonretentive fecal incontinence (FNRFI). 1 FDDs are common in children and represent a frequent reason for healthcare consultations and increased healthcare expenditure. 2–5 Childhood FC has a reported prevalence ranging from 0.7-29.6%. 6 The prevalence of FNRFI in the general pediatric population is estimated to be around 1%, although epidemiological studies on FNRFI are scarce. 7 FDDs are characterized by bothersome and embarrassing symptoms such as fecal incontinence and abdominal pain, which negatively impact quality of life of affected children and their families. 8–11 FDDs are diagnosed according to the symptom-based Rome criteria. In 2016, the Rome IV criteria were published, replacing the Rome III criteria published in 2006. 1,12 For FDDs, the changes in Rome IV were minor compared to Rome III. According to the new Rome IV criteria for FC, a distinction is made for young children between those who are toilet trained and those who are not. Furthermore, the time criterion has changed for both disorders, patients now need to fulfill the criteria for at least 1 month instead of 2 months. 1,12,13 The most recent systematic review on the prevalence of FC was published in 2011 and included studies with a variety of definitions for FC, including self-reported diagnoses, stool consistency, stool frequency, Rome II and Rome III criteria. 6 However, the use of different definitions of FC significantly affects epidemiological study outcomes and the authors refrained from performing a meta-analysis on these studies with heterogeneous methodologies. 14 Currently, a systematic review on the prevalence of FNRFI is lacking. A systematic review on the epidemiology of FC and FNRFI using uniform definitions, enabling a meta-analysis, could help to generate more insights into the worldwide epidemiology and potential risk factors. This could help to identify deficiencies in the current knowledge and to guide future research. Therefore, our aim was to perform a systematic review and meta-analysis on the epidemiology of FDDs in children according to the pediatric Rome III and Rome IV criteria. Our secondary aim was to evaluate geographical, age and sex distribution for both disorders and to report on factors associated with both disorders, such as environmental, lifestyle and psychosocial factors.