Weeks 11 – 13:
What does it test for? Increased risk for chromosomal abnormalities, such as trisomy 21, trisomy 18 and trisomy 13, as well as cardiac defects.
How is it given? First-trimester screening combines the results of blood tests and the findings of an ultrasound. Babies with chromosomal abnormalities, such as trisomy 21 (Down syndrome), can have increased fluid beneath the skin on the back of their neck, and the blood of women carrying babies with Down syndrome can have lower-than-usual levels of pregnancy-associated plasma protein A (PAPP-A) and higher-than-usual levels of human chorionic gonadotropin (hCG).
Using ultrasound, your provider can measure the thickness of the fluid beneath the skin at the back of your baby’s neck; this measurement is called the nuchal translucency (NT). Using a computer program, your provider will combine the NT measurement with the results of a blood test and your age. The result is then given as a fraction; 1 in 1,000 would mean that your baby has a 1 in 1,000 chance of being born with one of the conditions that the test looks for.
Weeks 15 – 20:
Multiple Marker Screening
What does it test for? Increased risk for chromosomal abnormalities, such as trisomy 21, trisomy 18 and trisomy 13, and neural-tube defects.
How is it given? To do a multiple marker screen, your provider takes a blood sample that looks at the levels of three (triple screen) or four (quad screen) substances in your blood which, if higher or lower than expected, can be signs of a birth defect, such as Down syndrome, trisomy 18, or a neural-tube defect. The blood sample goes to a laboratory, where levels of up to the following four substances can be measured: alpha-fetoprotein (AFP) (a protein made in the baby’s liver); estriol (a type of estrogen); human chorionic gonadotropin (a protein made by the placenta); inhibin A (a hormone). Based on the levels of these substances in your blood, the test will calculate the risk that your baby has Down syndrome, trisomy 18 or a neural-tube defect.
Is there a risk? No
What does it test for? Chromosomal abnormalities, such as trisomy 21, trisomy 18, trisomy 13, and single gene disorders, such as cystic fibrosis.
How is it given? To perform an amniocentesis, using ultrasound as a guide, the provider inserts a needle through the mother’s belly and uterus into the amniotic sac. (Some of your baby’s skin cells slough off into your amniotic fluid, and these cells contain your baby’s chromosomes and genes, which can be tested.) The provider then takes a small sample of amniotic fluid and sends it to a laboratory for analysis.
During an amniocentesis, women may feel slight pressure or cramping in the area where the fluid is removed. They may also have cramping, blood spotting or leakage of amniotic fluid afterward. (Women should call their provider if they experience bleeding, leakage of fluid or fever.)
Is there a risk? Yes. The risk of miscarriage is less than 1 in 300 to 500— the same as CVS.