What is the quad screen, and what does it test for?
The quad screen is a blood test usually done in the second trimester that measures four substances in your blood — AFP, hCG, estriol, and inhibin-A — to estimate your risk of certain chromosomal conditions and neural tube defects. It's a screening test, not a diagnostic one, so it doesn't tell you whether your baby has a condition. Instead, it combines your test results with your age, weight, ethnicity, and due date to calculate a personalized risk score. Many providers offer it as a routine option in prenatal care, though you can decline it if you'd rather not know.
The quad screen primarily screens for three things: Down syndrome (trisomy 21), trisomy 18 (Edwards syndrome), and neural tube defects such as spina bifida, where the spine doesn't close completely during development. Some versions also flag an increased chance of abdominal wall defects, where organs develop outside the belly. A high-risk result on any of these doesn't mean your baby has the condition — it means further testing is worth considering. A low-risk result lowers the likelihood but doesn't rule anything out completely, since no screening test catches every case.
When is the quad screen performed?
The quad screen is typically done between 15 and 20 weeks of pregnancy, with the most accurate window being 16 to 18 weeks. Testing outside that window is still possible, but the accuracy of the risk calculation depends on precise gestational dating, so your provider will confirm your dates before scheduling. If your dating is uncertain — from an irregular cycle or a late first ultrasound — your provider may adjust the timing or recommend a dating ultrasound first.
Some clinics offer the quad screen on its own; others combine it with first-trimester blood work and nuchal translucency ultrasound results into what's called integrated or sequential screening, which can improve detection. If you've already had first-trimester screening, ask your provider whether the quad screen adds meaningful information in your case or whether NIPT (noninvasive prenatal testing) might be a better fit. Timing and sequence vary by clinic, so this is worth a direct conversation rather than assuming one approach applies to everyone.
What are the four markers, and what does each one measure?
The quad screen measures four substances that naturally occur in your blood during pregnancy: alpha-fetoprotein (AFP), human chorionic gonadotropin (hCG), unconjugated estriol, and inhibin-A. Each one is produced by the placenta or your developing baby, and levels normally shift in a predictable pattern as pregnancy progresses. The lab compares your levels against what's expected for your exact stage of pregnancy, then looks at the pattern across all four markers together — no single marker is diagnostic on its own.
| Marker | What it is | What a high level may suggest | What a low level may suggest | |---|---|---|---| | AFP (alpha-fetoprotein) | Protein made by baby's liver | Neural tube defect, abdominal wall defect, twins | Down syndrome, trisomy 18, inaccurate dating | | hCG (human chorionic gonadotropin) | Placental hormone | Down syndrome | Trisomy 18 | | Unconjugated estriol | Hormone made by placenta and baby's liver | Usually not flagged alone | Down syndrome, trisomy 18 | | Inhibin-A | Hormone made by placenta | Down syndrome | Not typically used alone |
These four values are combined mathematically with your age, weight, ethnicity, and whether you have diabetes, since all of these affect normal marker levels. That combined calculation produces your personal risk score — something like '1 in 340' for Down syndrome, for example. A single marker being slightly outside the expected range is common and, on its own, usually isn't cause for concern. It's the overall pattern across all four markers, weighted against your individual factors, that determines whether your result is flagged as higher or lower risk.
What does a high-risk quad screen result mean?
A high-risk quad screen result means your calculated odds of a chromosomal condition or neural tube defect are higher than the general population's baseline — for example, a result of '1 in 90' instead of something like '1 in 700.' It is not a diagnosis, and most people who receive a high-risk result go on to have a baby without the condition being screened for. The result is a starting point for a conversation with your provider about what additional testing might clarify things, not a final answer.
Because the quad screen looks at population statistics rather than your baby directly, it produces both false positives — a high-risk flag when the baby doesn't have the condition — and false negatives, more often than newer tests like NIPT. If your result comes back high-risk, your provider or a genetic counselor will walk you through your specific numbers and the follow-up options that make sense for your situation, without you needing to interpret a percentage alone.
How accurate is the quad screen?
The quad screen is a screening tool, not a diagnostic one, and it's less precise than newer options like NIPT — it catches many, but not all, of the conditions it looks for, and it does produce false positives. Its accuracy also depends on correct gestational dating, since marker levels are compared against age-specific expected ranges; dating errors of even a week or two can shift a result. Carrying twins, having diabetes, or having a higher body weight can also affect marker levels and reduce the reliability of the calculated risk.
Because of these variables, many providers now offer NIPT as either an alternative or a follow-up to the quad screen, particularly for anyone whose result comes back high-risk or whose personal risk factors make interpretation harder. The quad screen still has a role — it's one of the few common screening tests that checks for neural tube defects, which NIPT does not screen for. Ask your provider which combination of tests fits your pregnancy and your own comfort with uncertainty.
Quad screen vs. NIPT — how do they compare?
The quad screen and NIPT are both blood tests, but they work differently and screen for different things: the quad screen measures placental and fetal proteins and includes neural tube defect screening, while NIPT analyzes fragments of fetal DNA and does not screen for neural tube defects. NIPT is generally considered more accurate for chromosomal conditions like Down syndrome and can be done slightly earlier, but it's also more expensive and isn't automatically covered by every insurance plan. Neither test is diagnostic — both require follow-up testing to confirm a result.
| | Quad screen | NIPT | |---|---|---| | What it measures | Four proteins and hormones in your blood | Fragments of fetal DNA in your blood | | Typical timing | 15–20 weeks | From about 9–10 weeks onward | | Screens for | Down syndrome, trisomy 18, neural tube defects | Down syndrome, trisomy 18, trisomy 13, sex chromosome differences | | Neural tube defect screening | Yes | No | | Type of result | Personalized risk score | Risk estimate, often reported as higher or lower chance | | Diagnostic? | No — screening only | No — screening only |
Your provider may recommend one, the other, or both, depending on your age, insurance coverage, personal risk factors, and how much information you want at each stage of pregnancy. There's no single right choice for everyone — it's a decision worth making with your provider rather than deciding from a chart alone.
What happens if my quad screen comes back positive?
If your quad screen comes back high-risk, your provider will typically recommend a genetic counseling appointment to walk through what the number means for your specific situation, followed by one or more follow-up options: a detailed ultrasound to look closely at your baby's anatomy, NIPT if you haven't already had it, or a diagnostic test like amniocentesis, which can confirm or rule out a chromosomal condition directly. None of these steps are mandatory, and you can decide how much follow-up feels right for you.
It's normal to feel frightened by a high-risk result, and it's also true that most of these results do not end in a diagnosis. Take the time you need before deciding on next steps, and lean on your provider or a genetic counselor to walk through your actual numbers rather than general statistics. If you experience heavy bleeding, severe abdominal pain, or fluid leaking after any follow-up procedure such as amniocentesis, call your provider the same day.