Objective To investigate the prevalence and associated factors of malocclusion among preschool children in Fengxian District, Shanghai, and to provide a scientific basis for local medical and public health departments to formulate targeted interventions.

Methods A cross-sectional study was conducted from September to December 2024. A total of 1 172 children aged 3‒5 years were recruited from 10 kindergartens across 5 townships in Fengxian District, using stratified cluster random sampling method. Clinical examinations were conducted to record occlusal parameters such as the relationship of the second deciduous molars, canine teeth relationship, anterior overbite and overjet, anterior and posterior crossbite, and crowded dentition. A questionnaire survey was performed to the parents to collect information on factors influencing malocclusion, including genetic predisposition, poor oral habits, dietary patterns, and sleep-related breathing conditions. Data were statistically analyzed using IBM SPSS Statistics 27 software, with chi-square test for categorical data and binary logistic regression analysis for influencing factors.

Results The overall prevalence of malocclusion among 3‒5-year-old children in Fengxian District of Shanghai was 70.73%, with no statistically significant differences between genders (P>0.05). Deep overbite accounted for the highest proportion (49.66%), followed by Angle Class Ⅱ malocclusion of the second deciduous molars (17.83% on the left, 19.11% on the right), midline deviation (13.23%), anterior crossbite (6.74%), and crowding of the maxillary and mandibular anterior teeth (16.38% for the maxillary dentition, 17.58% for the mandibular dentition). Multivariate analyses showed that habitual mouth breathing during the day (OR=0.509,P=0.009) and morning dry mouth upon waking (OR=1.455,P=0.023) were independent influencing factors for malocclusion.

Conclusion The prevalence of malocclusion is relatively high among preschool children in Fengxian District, Shanghai,with deep overbite being the dominant trait. Sleep-related breathing conditions (such as habitual mouth breathing during the day and morning dry mouth) is an important influencing factor for malocclusion. Public health authorities should strengthen oral health education and promote the correction of bad habits to reduce the prevalence of malocclusion and improve children’s oral health.