The objective of this report was to evaluate the effect of ultrasonographic (US) findings on pregnancy management in patients with marker chromosome (MC) aneuploidy ascertained through prenatal diagnosis. From 1989 throughJune 1993,15,522 prenatal diagnostic procedures were performed for accepted indications. Charts of patients with MC on amniocentesis or chorionic villus sampling (CVS) karyotype were evaluated with respect to US anomalies, pregnancy complications, and outcome. Nineteen cases of MC were identified. The prevalence of MC in our study was 0.12% (1:816 procedures). No significant difference between CVS and amniocentesis was found: 5/19 (26%) were CVS specimens, which is comparable to our CVS (3,259/15,522) case distribution. Three cases with incomplete records were excluded from the analysis. Four inherited MC cases were identified; 1 case had anencephaly. Of the 12 de novo MC cases 4 (33%) had abnormal US findings, and an additional 4 were found to have cytogenetic evidence for partial trisomy. Seven of these 8 abnormal de novo MC cases were terminated. MC aneuploidy is more common in pregnancies sampled for usual genetic indications than previously reported in pediatric series. Highresolution US may identify a major malformation not etiologically related to a MC inherited from a normal phenotypic parent. The association of the novo MC with US anomalies confers a poor prognosis, suggesting the expression of genetic imbalance from the accessory chromatin (partial trisomy). However, when US appears normal on initial and follow-up examinations, the chances for a normal-phenotypic newborn are high.