There are two types of procedures available for pregnant women in order to detect Down Syndrome :
Screening tests : these tests estimate the risk of the foetus having Down Syndrome
Diagnostic tests : these tests tell whether or not the foetus actually has the condition
The most commonly used screening tests are :
Triple Screen
Alpha-fetoprotein Plus
These tests measure quantities of various substances in the blood (Alpha fetoprotein, human chorionic gonadotropin and unconjugated estriol) and together with the woman's age, estimate her risk of having a child with Down Syndrome. These screening tests are typically offered between 15-18 weeks of gestation.
The procedures available for prenatal diagnosis of Down Syndrome are:
Chorionic Villus Sampling (CVS) : The placenta attaches to the wall of the uterus by little "fingers" or villi. A probe is inserted via the cervix and one of the villi removed and analyzed. Reserved for those women with a particularly high risk of recurrence in subsequent pregnancies. Risk to foetus - 2 to 3%.
Amniocentesis : A sample of amniotic fluid is obtained and analyzed for chromosome abnormality. The amniocentesis is performed by numbing a small area of skin on the lower abdomen with local anaestheticand drawing up the amniotic fluid with a needle and syringe, rather like sampling of blood. The risk of miscarriage following amniocentesis is about 1%.
Ultrasound Scanning : On examination of the foetus head ultrasound can identify the characteristic shape to detect Down Syndrome. The observation of thickened skin fold and a thigh bone that is shorter than normal combined with ultrasound detection of heart abnormalities (which are common in Down Syndrome) leads to 80% accuracy. Performed after 12 to 14 week stage.
Percutaneous Umbilical Blood Sampling (PUBS): Percutaneous Umbilical Blood Sampling (PUBS) is performed to test a pregnancy for a chromosome abnormality or other genetic conditions, depending on the patient's family history and availability of testing. A needle is inserted through the patient's abdomen while the foetus is still with in the uterus and a blood sample is withdrawn from the umbilical cord.
A karyotype or chromosome analysis is performed on the foetal blood sample. There is a 3-5% risk of complications which could include cramping, bleeding, leaking of amniotic fluid, abdominal tenderness, infection, onset of labour, slowing of foetal heart and foetal loss.
It can be done at 18 weeks of gestation and can be done up to delivery.
