Objective: To compare the detection rate by microarray analysis for chromosome abnormalities in a prenatal population to that of a neonatal population referred for diagnostic testing.Methods: Array comparative genomic hybridization (aCGH) analysis was performed for 151 prenatal cases and compared with the results from 1375 postnatal cases less than 3 months of age.Results: Two of 151 prenatal cases (1. 3%) showed a clinically significant cytogenetic abnormality. In contrast, of the 1375 postnatal cases studied, 11.4% showed a cytogenetic abnormality by aCGH. Many of these (40%) were referred for aCGH because of dysmorphic features, a clinical indication unlikely to be identified in the prenatal population.Conclusions: The chance of detecting a chromosome abnormality in a prenatal population that has already been screened by routine cytogenetics is approximately 1.3%. However, given that many of the abnormal array results in the neonatal population were among those with dysmorphic features as the primary indication for testing, which are not easily identifiable by ultrasound, offering prenatal testing by aCGH to a wider population would likely result in a higher detection rate.