First trimester screening consists of an ultrasound exam and a blood test performed between the 11th and 14th week of pregnancy. It is important to remember that this is just a screening test. It does not diagnose a problem in the pregnancy. It just lets the medical team know which pregnancies may have a higher risk for a particular condition.
During an ultrasound exam, the medical team estimates the gestational age of the baby and takes a measurement called nuchal translucency (NT). The NT measurement assesses the amount of fluid accumulated behind the baby’s neck. An increased NT measurement may be a sign of a chromosome abnormality or heart problem. The blood sample is analyzed to gauge levels of two substances normally found in the blood of all pregnant women: free beta-hCG and PAPP-A. The results of the ultrasound and the blood work are combined to estimate risk for Down syndrome (trisomy21), trisomy 18 and trisomy 13. Current data suggests that first trimester screenings can identify greater than 90 percent of babies with these conditions.
If the screening reveals an increased risk for one of these conditions you will be offered the opportunity to discuss the results with a genetic counselor. You may also have the option to have additional screenings or tests, such as a detailed ultrasound, chorionic villus sampling (CVS), or amniocentesis.
Since first trimester screening does not screen for open neural tube defects (such as spina bifida and anencephaly) it is often recommended that women pursue second trimester alpha-fetoprotein (AFP) screening and/or a detailed ultrasound as the pregnancy progresses.
Quad Screen
The quad screen (also called multiple marker screening or AFP4) is a blood test that screens for open neural tube defects (NTDs) such as spina bifida and for abdominal wall defects like gastroschisis. It also screens for chromosome changes such as Down syndrome (trisomy 21) and trisomy 18. It is important to remember that this is just a screening test. It does not diagnose a problem in the pregnancy. It just lets us know which pregnancies may have a higher risk for a particular condition.
Quad screening involves obtaining a sample of blood from the mother between 15 and 20 weeks of pregnancy and measuring the amount of alpha-fetoprotein (AFP), human chorionic gonadotrophin (hCG), unconjugated estriol (uE3), and dimeric inhibin-A (DIA) present. A woman’s age, weight, ethnicity, diabetic status, gestational age of the baby, and whether there are twins are all used in the risk assessment calculation. An elevated level of AFP in the mother’s blood could indicate an increased chance of having a baby with a NTD or abdominal wall defect. A refined estimate of a woman’s risk for carrying a child with a chromosome change is also possible using this screening. The risk for Down syndrome is increased when the levels of AFP and uE3 are lower and the levels of hCG and DIA are higher than expected.
If screening reveals an increased risk for a particular condition, you will be offered the opportunity to discuss the results with a genetic counselor. You may also have the option to have additional screening or testing, such as a detailed ultrasound or amniocentesis.