First Trimester Screening Test

First trimester screening is a prenatal test that offers early information about a baby's risk of certain chromosomal conditions, specifically, Down syndrome (trisomy 21) and extra sequences of chromosome 18 (trisomy 18).

First trimester screening, also called the first trimester combined test, has two steps:

• A blood test to measure levels of two pregnancy-specific substances in the mother's blood — pregnancy-associated plasma protein-A (PAPP-A) and human chorionic gonadotropin (HCG)
• An ultrasound exam to measure the size of the clear space in the tissue at the back of the baby's neck (nuchal translucency)

Typically, first trimester screening is done between weeks 11 and 14 of pregnancy.

Using your age and the results of the blood test and the ultrasound, your health care provider can gauge your risk of carrying a baby with Down syndrome or trisomy 18.

If results show that your risk level is moderate or high, you might choose to follow first trimester screening with another test that's more definitive.

Second Trimester Screening Test

The quad screen — also known as the quadruple marker test, the second trimester screen or simply the quad test — is a prenatal test that measures levels of four substances in pregnant women's blood:

• Alpha-fetoprotein (AFP), a protein made by the developing baby
• Human chorionic gonadotropin (HCG), a hormone made by the placenta
• Estriol, a hormone made by the placenta and the baby's liver
• Inhibin A, another hormone made by the placenta

Ideally, the quad screen is done between weeks 15 and 18 of pregnancy — during the second trimester. However, the procedure can be done up to week 22.

The quad screen is used to evaluate whether your pregnancy has an increased chance of being affected with certain conditions, such as Down syndrome or neural tube defects. If your risk is low, the quad screen can offer reassurance that there is a decreased chance for Down syndrome, trisomy 18, neural tube defects and abdominal wall defects.

If the quad screen indicates an increased chance of one of these conditions, you might consider additional screening or testing.

Source
https://www.mayoclinic.org/tests-procedures/quad-screen/about/pac-20394911