Screening tests are laboratory tests that help pregnant women identify whether they have an increased risk for a condition or disease, so that additional measures can be taken. Two types of screening tests are a first trimester combined test and an integrated screening test.  A first trimester combined test merges the results from a blood test and a nuchal translucency ultrasound.  An integrated screening test includes the first trimester combined test and a second trimester test that tests for abnormal levels of alpha fetoprotein, estriol, HCG and inhibin A in blood.  

 

A blood test measures the levels of pregnancy-associated plasma protein-A (PAPP-A) and the pregnancy hormone known as human chorionic gonadotropin (HCG). Babies with Down syndrome tend to have high levels of hCG and low levels of PAPP-A.

A nuchal translucency (NT) ultrasound is a test which assesses whether your baby is likely to have Down syndrome. It is measured using an ultrasound when your baby is between 11 weeks and 13 weeks plus six days old. All babies have some fluid, but many babies with Down syndrome have an increased amount. An NT measurement of up to 2mm is normal at about 11 weeks, and up to about 2.8mm by 13 weeks and six days.  If there is more fluid than normal, there is a chance that the baby has Down syndrome.

 

If other tests indicate that the baby is extremely likely to have Down syndrome, there are several other types of tests that can be performed a cell-free fetal DNA test (a test that checks for fetal DNA in the mother’s blood), an amniocentesis (an invasive test that analyses amniotic fluid from around the baby), chorionic villus sampling (DNA of cells taken from the placenta is analysed) and finally a cordocentesis or percutaneous umbilical blood sampling (blood from an umbilical vein is analyzed, this test has a significant  risk of causing a miscarriage).