First Trimester: Down Syndrome Screen
It is possible during the early stages of pregnancy to estimate the possibility of a mother having a baby affected by Down syndrome using blood tests, sometimes in combination with ultrasound measurements. These tests may also indicate the possibility of other chromosome disorders, such as Edwards syndrome. This type of screening provides an opportunity to assess the possibility of these conditions being present without performing more invasive procedures, such as chorionic villus sampling or amniocentesis. Only those women whose screening results indicate that they are in a higher risk group are currently counseled about having further tests.
The first trimester Down syndrome screen is usually performed between 11 and 14 weeks of pregnancy and includes the measurement of blood levels of pregnancy associated plasma protein A (PAPP-A) and either free or total human chorionic gonadotrophin (hCG). In addition, an ultrasound scan called nuchal translucency is done. This involves taking a measurement of the thickness of the skin and tissue at the back of the baby's neck. The results from these tests will be combined and used to calculate the chance of the baby having Down syndrome or other chromosome abnormality.