There are several parameters used to calculating the risk for delivering a baby with Down syndrome. These factors include:





1. The development of Alpha-fetoprotein in the part of the womb called the yolk sac and in the fetal liver, and whether or not some amount of AFP gets into the mother's blood. During neural tube defects, the skin of the fetus is not intact and so larger amounts of AFP are measured in the mother's blood. During Down syndrome, AFP is present in decreased amounts in the mother's blood, presumably because the yolk sac and fetus are smaller than usual.





2. Estriol is a hormone produced by the placenta, using ingredients made by the fetal liver and adrenal gland. Estriol is also present in decreased amounts during a Down syndrome pregnancy. This test may not be included in all screens, depending on the laboratory.





3. The human chorionic gonadotropin (HCG) hormone is produced by the placenta, and is used to test for the presence of a pregnancy. A specific smaller part of the hormone, called the beta-subunit, is present in increased amounts during Down syndrome pregnancies.





4. Inhibin A is a protein secreted by the ovary, and is designed to inhibit the production of the hormone FSH by the pituitary gland. The level of inhibin A is increased in the blood of mothers of fetuses with Down syndrome.





5. PAPP-A, which stands for Pregnancy-Associated Plasma Protein-A, is produced by the covering of the newly fertilized egg. In the first trimester, low levels of this protein are observed in Down syndrome pregnancies.





More parameters used for calculating risk for having baby with Down syndrome are:


6. Age of the fetus (gestational age);


7. Mother’s age and,


8. Size of a collection of fluid at the nape of the fetal neck, called the “nuchal translucency”.








It is important to note that these parameters are only predictive and not diagnostic. This means that they are used only for calculating the risk and not for confirming the Down syndrome. Down syndrome can be confirmed with amniocentesis.





Amniocentesis is used to collect amniotic fluid, the liquid that is in the womb. A needle is inserted through the mother's abdominal wall into the uterus, using ultrasound to guide the needle. Approximately one ounce of fluid is taken for testing. This fluid contains fetal cells that can be examined during chromosome tests. It takes about 2 weeks to determine if the fetus has Down syndrome or not. Amniocentesis is not recommended before the 14th week of pregnancy due to a higher risk of complications and loss of pregnancy. Amniocentesis is recommended for women with a risk of having a child with Down syndrome of 1 in 250 or greater and you (with a risk factor of 1 in 1715) don’t belong in this category.