Children with sex chromosome abnormalities (SCA) are known to be at risk for developmental delays. These risks were identified 2 decades ago by seven international research groups who prospectively followed children ascertained after birth. Subsequently, some of these investigators suggested the course of prenatally identified children with SCA may be different from children in earlier studies. The first such evidence was published by Robinson et al. [1992: Am J Med Genet 44:365–368], who compared 20 prenatally diagnosed children to the original postnatally diagnosed cohort. The following report presents an update and expansion of that research and includes 51 children and adolescents prenatally diagnosed with SCA, now 7–18 years of age. Results confirm that this cohort of prenatally diagnosed children has a milder developmental course than children ascertained postnatally. The study provides new information to health professionals counseling families faced with prenatal diagnosis of SCA. © 2002 Wiley-Liss, Inc.