The second-trimester screening test, also known as the “Quad Screen,” is a prenatal test that looks for chromosomal abnormalities, neural tube defects (defects of the brain, spine, and/or spinal cord), and abdominal wall defects (abnormal openings in the abdomen) in developing fetuses.
When Does the Quad Screen Happen?
Between 15-22 weeks of pregnancy (ideally between 16-18 weeks).
What is the Second Trimester Quad Screen?
It’s a blood test that measures 4 different substances.
These Four Substances Are:
1. Human Chorionic Gonadotropin (hCG): This famous pregnancy hormone is made by the placenta.
Note: The first trimester screen measures hCG levels, as well.
2. Alpha-Fetoprotein (AFP): A protein made by the baby’s liver.
3. Estriol: A hormone made by both the placenta & the baby’s liver.
4. Inhibin A: Another hormone made by the placenta.
Insider Info: There’s another second trimester screening test called the “Triple Screen” which measures three substances: alpha-fetoprotein (AFP), human chorionic gonadotropin (hCG), and estriol. It’s not as accurate as the “Quad Screen,” though. If given a choice between the two, ask for the Quad Screen.
What Will the Quad Screen Tell Me?
If will tell you your odds of having a baby with Down Syndrome (Trisomy 21), Edwards Syndrome (Trisomy 18), a neural tube defect, or an abdominal wall defect. Get Wise(r) about these conditions below:
- Down Syndrome is a condition characterized by intellectual disability (a low IQ) and certain physical features (such as short stature, small ears, almond-shaped eyes that slant upwards, and a protruding tongue). The most common cause of Down Syndrome is an extra copy of chromosome 21. Patients with Down Syndrome have 3 copies of chromosome 21 in their cells (vs. the usual 2 copies).
- Edwards Syndrome is caused by an extra chromosome 18. In this case, patients have 3 copies of chromosome 18 in their cells. This leads to a host of problems, including severe intellectual disability, a low birth weight, heart problems, a small, abnormally-shaped head, deformed feet, and clenched fists with overlapping fingers. Sadly, only 5-10% of babies with Edwards Syndrome live past their first year of life.1
Reality Check: Though serious, Edwards Syndrome is extremely rare.
- Neural Tube Defects: Neural tube defects are defects of the brain, the spine, and/or the spinal cord.
Examples Include: Spina Bifida (a spinal cord issue) and Anencephaly (a disease in which the brain doesn’t fully develop).
- Abdominal Wall Defects: An abdominal wall defect is an abnormal opening in the abdomen, through which digestive organs (think: the stomach and intestines) can protrude.
Reality Check: Abdominal wall defects are rare and can usually be corrected with surgery.
Is the Second Trimester Screening Test Accurate?
Yes, it’s fairly accurate. The second trimester screen correctly identifies about 80% of Down Syndrome Cases, 80% of Trisomy 18 cases, and 85% of neural tube defects.2
Caveat: The second trimester screen has a slightly higher rate of false positives (in which the test is wrongly positive) than other prenatal screening tests (like the First Trimester Screening Test and the NIPS Test). Therefore, if your Quad Screen comes back positive, try not to go down a rabbit-hole of worry. A follow-up diagnostic test (chorionic villus sampling or a “genetic” amniocentesis) will be done to confirm (or refute) the result.
False Positive Results Can Occur for a Variety of Reasons With the Quad Screen. These Include:
- Your due date is off.
- You’re having more than one baby.
- Your baby is a product of in vitro fertilization (IVF).
- You’re a smoker.
- You have diabetes or another medical condition that can throw off the results.
Which Prenatal Screening Test Is Best: The Second Trimester Screen, The First Trimester Screen, or the NIPS Test?
Although the second trimester screen has its advantages (e.g. it tests for neural tube defects and abdominal wall defects in addition to Down Syndrome and Edwards Syndrome), many couples elect to do the first trimester screen or the NIPS test instead because they’re performed earlier in the pregnancy. Get Wise(r) about the similarities and differences between the First Trimester Screen, the Second Trimester Screen, and the NIPS Test, here.
And…Breathe…
