Trisomy 21 Test During Pregnancy. Use standard operating procedures to decide whether to accept prenatal diagnoses without postnatal confirmation (e.g. The test is validated for women with a positive first trimester risk assessment, is applicable in singleton and recently also in twin pregnancies, and cannot detect mosaic trisomies.
Trisomy 18 and trisomy 13 occur within fewer pregnancies than trisomy 21. The results of the blood test, the nuchal translucency measurement and the mother's age are used to estimate the risk for down syndrome and trisomy 18. However, when trisomy 21 occurs, the chromosome pairs do not split.