Complete trisomy 21 vs translocation Down syndrome: a comparison of modes of ascertainment.

OBJECTIVE

To compare the indications for invasive prenatal testing resulting in the detection of translocation Down syndrome and complete trisomy 21.

STUDY DESIGN

This case control study was based on a large amniocentesis and chorionic villi samples database (n = 534,795). All specimens with translocation Down syndrome (n = 203) comprised the translocation group and were compared with a maternal age-matched group (4 to 1, n = 812) in which complete trisomy 21 was detected. Women with a normal karyotype were randomly selected (n = 812) and served as controls. Indications for invasive testing were compared among the 3 paired groups using χ(2) analysis.

RESULTS

There were no differences in the incidence of abnormal first- and second-trimester screening tests between the translocation Down syndrome and the complete trisomy 21 groups. History of prior aneuploidy was significantly more frequent in the translocation Down syndrome group, as compared with either complete trisomy 21 fetuses or normal controls.

CONCLUSION

Fetuses with translocation Down syndrome present with the same screening abnormalities as fetuses with complete trisomy 21.