A prenatal diagnostic test can be offered to the couple e.g. when the screening result indicates a higher risk or when there is a known disease in the family. The prenatal diagnostic tests include e.g. ultrasound scan and genetic tests on material from chorionic villus sample or amniocentesis. A prenatal genetic test can e.g. be a chromosome analysis testing for chromosome disorders or a DNA based test testing for e.g. a specific genetic disease known in the family. But not all genetic diseases can be excluded.
Pregnant women are offered two prenatal screening tests:
1) A first trimester screening based on a blood sample at week 8-13 of gestation measuring PAPP-A (Pregnancy associated protein-A) and free hCG (human Chorionic Gonadotropin), and an ultrasound scan at week 11-14 of gestation where the nuchal thickness is measured. A combined risk for Down syndrome is estimated. If a high combined risk is found, an invasive prenatal test (often a chorion villus sample) is offered.
2) A second trimester ultrasound scan at week 18-20 of gestation. The fetus will be examined for structural abnormalities and developmental defects. If for example a structural malformation is found, an invasive prenatal test (often an amniocentesis) is offered. At the second trimester ultrasound scan the localization of the placenta is also described.