Two phenotypically abnormal liveborns in whom trisomy 16 mosaicism was diagnosed prenatally by amniocentesis are described. Analysis of a percutaneous umbilical blood sample in one case revealed a normal chromosomal complement. Ultrasound examinations performed at the time of amniocentesis were normal. Serial sonography during the late second and third trimesters demonstrated progressive intrauterine growth retardation (IUGR) in both fetuses and a cardiac defect in one. At birth, both infants had dysmorphic features and multiple congenital anomalies. Trisomy 16 mosaicism was confirmed postnatally in both infants in skin fibroblasts; however, peripheral blood samples contained only chromosomally normal cells. The two mosaic trisomy 16 cases described in this report, together with the five confirmed cases reported previously, demonstrate the need for caution in the counselling of patients when trisomy 16 mosaicism is diagnosed prenatally in amniotic fluid samples. Such cases potentially can result in the birth of dysmorphic infants with significant birth defects, growth retardation, and possible developmental disabilities.
The predicted incidence of liveborn monozygotic trisomy 18 twins is one per million births. The first case of liveborn monozygotic trisomy 18 twins was reported in 1989 and we report a second case in which striking phenotypic discordance existed. The probability of monozygotic trisomy 18 twinning and the mechanisms for phenotypic discordance in trisomic twins is discussed. Case report A 26 year old white woman, gravida 2, para 1-0-0-1, with a twin pregnancy, was admitted to hospital at 27 weeks' gestation because of premature rupture of membranes and preterm labour. Ultrasonographic examination at this time showed discordant fetal measurements: biparietal diameters 71 and 60 mm, and femur lengths 48 and 44 mm (values for twin A and twin B, respectively). Twin A, who had fetal measurements appropriate for gestational age, had a two vessel umbilical cord and an umbilical artery blood velocity systolic/diastolic ratio (S/D ratio)= 2 5-3@1. Twin B had a "lemon shaped appearance of the fetal head" (fig la) and a cystic spine defect at the level of the lumbar vertebrae consistent with a
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