AbstractThe frequency of triploid conceptuses in man is estimated to be approximately 1%; most are aborted during the first trimester. There are reports of 31 triploid fetuses which survived past the 24th week of gestation, including several infants born at term. We are unaware of a report of the prenatal diagnosis of triploidy.A typical clinical syndrome has been delineated in nonmosaic triploid fetuses and infants, including intrauterine growth retardation, syndactyly, eye and ear abnormalities, hypogonadism in males, and hydatidiform placenta.A 32‐year‐old primigravida presented with small uterine size at 16 weeks gestation. Subsequently the uterus grew very slowly. Ultrasound examination at 23 weeks revealed a fetal head size consistent with 20 weeks gestation and a thoracic diameter consistent with 14 weeks. Amniocentesis was done for karyotype analysis and α‐fetoprotein determination. Amniography showed a small fetus with obvious disproportion between head and trunk. Abortion was induced with prostaglandin. The fetus had features consistent with the triploidy syndrome, and the amniotic fluid cells were later found to have a 69,XXY karyotype. Chromosomal polymorphisms of the fetus and parents indicated that the supernumerary haploid set of chromosomes originated from failure of the second meiotic division in the mother.This case illustrates the way in which the combined use of amniocentesis, ultrasonography, and radiographic techniques can be advantageous in prenatal diagnosis and genetic counseling, especially when the patient is relatively late in pregnancy so that it is necessary to obtain a rapid dia