OBJECTIVES: The aim of this study was to estimate the prevalence of specific neurodevelopmental disorders in children prenatally believed to have isolated mild ventriculomegaly in the second trimester of pregnancy to optimize the counselling process.METHODS: This was a nationwide register based study including all singleton pregnancies which had a first and second trimester scan in the period 1 January 2008 to 1 October 2014, identified through the Danish Fetal Medicine Database and from local clinical databases. All fetuses prenatally believed to have isolated mild ventriculomegaly (measurement of the atrium of the lateral ventricles between 10.0-15.0 mm) diagnosed prenatally between 18-22 weeks of gestation were followed-up through national patient registers until the age of 2 to 7 years. We obtained information about the diagnoses of intellectual disability, cerebral palsy, autism spectrum disorders, epilepsy and impaired psychomotor development from national registers. Results Among a cohort 292,046 of fetuses, 133 cases were found to have apparent isolated mild at the second trimester scan for fetal malformations. In eleven cases, long-term follow-up was not possible due to termination of pregnancy, spontaneous miscarriage, neonatal death and loss to follow-up. Among 122 live born children followed up for between 2-7 years, 15 children were identified with additional abnormalities while 107 children were postnatally confirmed with isolated mild ventriculomegaly. Of the 107 children, a neurodevelopmental disorder diagnosis was registered in 6 (5.6%) cases corresponding to an OR of 2.64 (95% CI 1.16-6.02) compared to the reference population. The diagnoses were autism spectrum disorders, epilepsy and impaired psychomotor development. None of the children were diagnosed with intellectual disability or cerebral palsy.CONCLUSIONS: Our results show that a diagnosis of confirmed isolated mild ventriculomegaly was associated with an increased risk of neurodevelopmental disorders compared to the reference population, but the absolute risk was low and there were no cases of intellectual disability or cerebral palsy. This article is protected by copyright. All rights reserved.