Pregnancy

The Quad Screen: A Second-Trimester Check

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The quad screen is a second-trimester blood test, done between 15 and 20 weeks, that measures four markers to estimate the chance of Down syndrome, trisomy 18, and open neural-tube defects such as spina bifida. It is a screen, not a diagnosis: a higher-chance result leads to an offer of detailed ultrasound or diagnostic testing.

Last updated: July 2026

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What does the quad screen actually test?

The quad screen measures four substances in the blood - alpha-fetoprotein (AFP), human chorionic gonadotropin, estriol, and inhibin A 1. A laboratory blends these four markers with your age and gestational timing into estimated odds for specific conditions. According to NICE antenatal guidance, the quadruple test screens for Down syndrome and is offered when someone has passed the first-trimester window 1. The AFP marker also helps flag open neural-tube defects such as spina bifida, which is why the quad remains useful even where NIPT is available 2. Each marker on its own means little; the pattern across all four is what matters.

When is the quad screen done, and who gets it?

Timing is what sets the quad screen apart: it is a second-trimester test, usually offered between 15 and 20 weeks 1. According to NICE, the quadruple test is the recommended option for people who book too late for the first-trimester combined screen, which closes around 14 weeks 1. It can be offered to anyone who is pregnant, regardless of age, though the estimated chance rises with maternal age 3. Some choose it because they missed the earlier window; others because they prefer a single blood test without an ultrasound component. Discussing timing at your first prenatal appointment helps you land in the right window 2.

How should you read a quad screen result?

A quad screen result is reported as odds, such as 1 in 250, describing the chance of a condition rather than confirming it 1. Results are usually labeled screen-positive or screen-negative against a threshold, but a screen-positive result is not a diagnosis. Many people with a higher-chance quad result have healthy babies, because the test is designed to over-refer rather than miss cases 2. When a result is higher-chance, the next step is typically a detailed ultrasound arranged through early prenatal care, NIPT, or diagnostic testing such as amniocentesis 1. According to prenatal-care guidance, a screen-positive quad is a reason for more information, not alarm 2.

How does the quad screen compare with NIPT?

The quad screen and NIPT answer overlapping questions in different ways 1. NIPT is more sensitive for the common trisomies and can be done earlier, from about 10 weeks, but it does not screen for neural-tube defects 1. The quad screen is less sensitive for chromosomal conditions, yet its AFP marker adds neural-tube coverage that NIPT lacks 2. Interpretation also shifts across the reproductive lifespan - the baseline chance a screen weighs is higher near the perimenopausal transition in the forties than in the teens 3. Some people who used IVF also review genetic testing of embryos results alongside these screens 2.

When a quad screen result needs a specialist

A screen-positive quad result, an unusually high or low AFP, or an abnormal ultrasound are reasons to involve a maternal-fetal medicine specialist or genetic counselor 2. These clinicians can explain your specific odds, arrange a detailed anatomy scan, and discuss NIPT or diagnostic testing. Staying connected through scheduled prenatal care makes any follow-up easier to coordinate 2. Gale can help you prepare questions for that visit.

Common questions

The quad screen measures alpha-fetoprotein (AFP), human chorionic gonadotropin, estriol, and inhibin A. A laboratory combines these four with your age and gestational timing to estimate the chance of certain conditions. No single marker is meaningful on its own.

Yes, in several situations. It is offered to people who book too late for first-trimester screening, and its AFP marker screens for open neural-tube defects, which NIPT does not cover. Some programs and insurers also use it as a first-line option.

Not necessarily. A screen-positive result raises the estimated chance but is not a diagnosis, and many people with a positive result have healthy babies. The usual next steps are a detailed ultrasound and, if wanted, NIPT or diagnostic testing to clarify.

It is a second-trimester test, usually between 15 and 20 weeks, and often around 16 weeks. If you are past the first-trimester combined screening window, the quad screen is a common alternative your clinician can offer.

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When a quad screen result needs follow-up

  • —A screen-positive quad result is a reason to arrange counseling before deciding on further testing
  • —An unusually high AFP is a reason to discuss a detailed anatomy ultrasound with your obstetric clinician
  • —An abnormal finding on a follow-up scan is a reason to seek review with a maternal-fetal medicine specialist
  • —Anxiety about a result that interferes with daily life is a reason to reach out to your clinician for support

This article is general health education about prenatal screening, not medical advice. Interpreting a quad screen result and choosing any follow-up testing are decisions to make with an obstetric clinician, maternal-fetal medicine specialist, or genetic counselor.

References

  1. 1.National Institute for Health and Care Excellence (2024). Antenatal care (NG201). National Institute for Health and Care Excellence (NICE). link ✓Defines the quadruple (quad) test - measuring AFP, hCG, estriol, and inhibin A - as a second-trimester screen for Down syndrome offered between about 14 and 20 weeks for those past the first-trimester combined window, estimating a chance rather than diagnosing.
  2. 2.Office on Women's Health (U.S. HHS) (2025). Prenatal care. Office on Women's Health (womenshealth.gov), U.S. HHS. link ✓Patient-facing confirmation that second-trimester blood screening, including the AFP marker, screens for conditions such as neural-tube defects and Down syndrome, with a higher-chance result followed by diagnostic testing discussed with an obstetric clinician.
  3. 3.American College of Obstetricians and Gynecologists / American Society for Reproductive Medicine (2014). Female age-related fertility decline. Committee Opinion No. 589. Obstetrics & Gynecology. doi:10.1097/01.AOG.0000444440.96486.61 ✓Documents that the baseline chance of fetal chromosomal differences rises with maternal age.

3 sources, numbered by first appearance. General health information, not medical advice. AI-assisted editorial content — every citation independently verified. Editorial policy

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