A retrospective study was performed comparing the antenatal ultrasound (US) scan features with post‐mortem findings in 20 triploid fetuses surviving beyond the first trimester. US scans showed a range of abnormalities including oligohydramnios (60 per cent of cases), intra‐uterine growth retardation (IUGR, 55 per cent), central nervous system (CNS) abnormalities (45 per cent), and placental abnormalities (55 per cent). There were obstetric complications in 30 per cent of cases. Post‐mortem examinations confirmed these findings but also demonstrated additional anomalies including facial dysmorphism, limb abnormalities (syndactyly, talips equinovarus and thigh muscle hypertrophy), and anomalies of cardio‐vascular, respiratory and genito‐urinary system. The placenta was normal in 16 per cent of cases, enlarged with cystic change in 47 per cent, small in 32 per cent and infarcted in 5 per cent of cases. Various factors including oligohydramnios, abnormal fetal position, less sophisticated equipment in mid‐1980s and relative inexperience of early observers contributed to the reduced sensitivity of US scans in diagnosis of various defects. However, triploidy should be considered in all cases showing early second‐trimester IUGR with or without oligohydramnios, a major CNS abnormality and/or an enlarged placenta with or without changes of partial hydatidiform mole. As triploidy is associated with increased obstetric complications, fetal karyotyping should be offered to confirm the diagnosis so that appropriate obstetric intervention can be instituted. Copyright © 1998 John Wiley & Sons, Ltd.