Combined screening

Trisomies 18 and 13 are the second and third most common chromosomal abnormalities after trisomy 21.

At 11-13 weeks the relative prevalence of trisomies 18 and 13 to trisomy 21 are about 1 to 2.5 and 1 to 7, respectively.

All three trisomies are associated with increased maternal age, increased fetal NT and decreased maternal serum PAPP-A.

A beneficial consequence of first-trimester combined screening for trisomy 21 is the early diagnosis of trisomies 18 and 13. At a false positive rate of 3% the detection rate of trisomy 21 is 90% and of trisomies 18 and 13 is about 75%.