The first trimester Down screening test (FTS) combines biochemical parameters in maternal blood with ultrasound parameters of the fetus. The biochemical tests are PAPP-A (Pregnancy Associated placental protein-A) and free beta-HCG (Human Chorionic Gonadotrophin). These values are combined with ultrasound measurements of the nuchal translucency (NT), a translucent spot in the neck of the fetus and crown-rump length (CRL) of the fetus. The blood parameters PAPP-A and free beta-HCG can be performed in week 9-11, whereas the NT should be performed in week 12-13. The 1st trimester Down screening is not a diagnostic test, but a screening test which only indicates the risk for Down syndrome.
The sensitivity and specificity of the FTS test are low:
- Sensitivity: more than 20 % of the fetal trisomies escape detection by the FTS (False-negative test): this means the FTS test is normal, but the baby has a trisomy.
- Specificity: More than 95 % of the FTS tests are false-positive : this means the FTS test is abnormal, but the baby has no trisomy.
