Trisomy 20 mosaicism and nonmosaic trisomy 20: a report of 2 cases.

BACKGROUND

Together with chromosome 19, chromosome 20 belongs to group F, the group of small metacentric chromosomes. Trisomy 20 mosaicism is one of the most frequent chromosomal mosaicisms, representing approximately 16% of prenatally diagnosed cases. In nonmosaic trisomy 20, the usual findings are severe and manifold. Only 3 cases in the literature involved fetuses surviving past the first trimester.

CASES

In case 1, a 42-year-old woman presented in her sixth pregnancy; she had had 4 vaginal deliveries of term infants and a miscarriage. Both her familial and personal genetic histories were unremarkable. Genetic amniocentesis was performed in the 18th gestational week for advanced maternal age. Sample analysis revealed a normal, male karyotype in 27 mitoses, while 4 were trisomy 20 (46,XY [27]/47,XY, +20 [4]). In the 37th gestational week a live, immature, male infant weighing 1,730 g was delivered. Chromosomal investigation of the newborn's blood sample did not reveal trisomy 20 but a normal male karyotype. In case 2, a healthy 37-year-old nullipara underwent amniocentesis at the 18th week of pregnancy for advanced maternal age. Amniotic fluid cell karyotype revealed trisomy 20 (47,XX, +20). Ultrasonography performed simultaneously with genetic amniocentesis showed slightly shortened fetal long bones, detectable narrowing of the cranium in the region of the frontal bone, lateral ventricles of 10 mm in width bilaterally, echogenic bowel and polyhydramnios. Abortion was induced in the 23rd week of pregnancy, and a 490-g female fetus was delivered.

CONCLUSION

Based on these 2 well-documented, prenatally diagnosed cases, as far as genetic counseling is concerned, nonmosaic trisomy 20 is much less challenging than its mosaic form since the prognosis is uniformly poor in the former.