Percutaneous umbilical blood sampling (PUBS) is increasingly utilized for cytogenetic assessment of the fetus. In the present study, outcome of 109 PUBS-derived fetal blood samples were analyzed according to mode of ascertainment: ultrasound-detected anatomic anomalies or intrauterine growth retardation (IUGR). Abnormal chromosome complements were found in 6 of 47 (12.8%) fetuses with anomalies but in only 1 of 62 (1.6%) fetuses with IUGR alone. Perinatal loss in fetuses with anatomic anomalies was 64.4%, compared with only 1.6% in fetuses with IUGR alone. Mode of patient ascertainment is an important factor that must be considered in determining procedure-related risks for PUBS.