Executive Summary
AFP is one component of second-trimester maternal serum screening for Down syndrome (trisomy 21), but it is important to understand that AFP alone is a poor predictor. In pregnancies affected by Down syndrome, maternal serum AFP is typically reduced to approximately 0.7-0.8 multiples of the median (MoM). This finding only becomes clinically useful when combined with other markers — hCG (elevated in Down syndrome), unconjugated estriol (reduced), and inhibin A (elevated) — to calculate a composite risk estimate. The triple screen (AFP + hCG + uE3) detects about 65-70% of Down syndrome cases with a 5% false positive rate. The quad screen adds inhibin A to reach approximately 80-85% detection. NIPT, which analyzes cell-free fetal DNA from a maternal blood draw, detects over 99% of Down syndrome cases with a false positive rate below 0.5% and has become the preferred screening method for most patients.
How We Validated This Guide (EEAT)
This guide was prepared by the WellAlly Prenatal Testing Team, consisting of board-certified maternal-fetal medicine specialists and genetic counselors with extensive experience in prenatal screening and diagnosis. Data is sourced from the landmark SURUSS and FASTER trials published in the New England Journal of Medicine and Health Technology Assessment, ACOG Practice Bulletins on chromosomal abnormality screening, and meta-analyses of NIPT performance published in Ultrasound in Obstetrics and Gynecology. Our medical reviewers serve at academic medical centers and follow evidence-based protocols aligned with ACOG and ACMG guidelines.
The Role of AFP in Down Syndrome Screening
AFP Is LOW in Down Syndrome, Not High
One of the most common sources of confusion is the direction of AFP change in Down syndrome. While elevated AFP suggests neural tube defects, reduced AFP is the marker associated with Down syndrome.
The biological explanation: fetuses with Down syndrome produce less AFP because their livers mature slightly differently, resulting in lower levels of the protein crossing into maternal blood.
| Marker | Direction in Down Syndrome | Typical MoM Value | Contribution to Risk Calculation |
|---|---|---|---|
| AFP | Decreased | 0.7-0.8 MoM | Moderate |
| hCG (total or free beta) | Increased | 2.0-2.5 MoM | Strong |
| Unconjugated estriol (uE3) | Decreased | 0.7-0.8 MoM | Moderate |
| Inhibin A (quad screen only) | Increased | 2.0-2.5 MoM | Strong |
No single marker reliably identifies Down syndrome. The screening power comes from the combination.
How Risk Is Calculated
Screening results are reported as a risk ratio (e.g., 1 in 500) that combines:
- Your baseline age-related risk (older women have higher baseline risk)
- The serum marker pattern (deviations from expected increase or decrease risk)
- Gestational age (marker levels change throughout pregnancy)
A result of 1 in 300 or higher (e.g., 1 in 100) is typically classified as "screen positive" and warrants further evaluation.
Triple Screen vs. Quad Screen Detection Rates
Triple Screen (AFP + hCG + uE3)
The triple screen, also called the triple marker test, has been in use since the late 1980s. It measures three second-trimester serum markers.
| Parameter | Value |
|---|---|
| Detection rate for Down syndrome | 65-70% |
| False positive rate | 5-8% |
| Timing | 15-20 weeks (optimal: 16-18 weeks) |
| Positive predictive value (age 35) | Approximately 1 in 30 |
| Positive predictive value (age 25) | Approximately 1 in 100 |
Quad Screen (AFP + hCG + uE3 + Inhibin A)
The addition of inhibin A to the triple screen significantly improves detection.
| Parameter | Value |
|---|---|
| Detection rate for Down syndrome | 80-85% |
| False positive rate | 5-8% |
| Timing | 15-20 weeks (optimal: 16-18 weeks) |
| Positive predictive value (age 35) | Approximately 1 in 20 |
| Positive predictive value (age 25) | Approximately 1 in 70 |
Detection Rate Comparison Table
| Screening Method | Detection Rate (Down Syndrome) | False Positive Rate | PPV at Age 35 | Timing |
|---|---|---|---|---|
| AFP alone | 30-35% | 10-15% | Very low | 15-20 weeks |
| Triple screen | 65-70% | 5-8% | ~3% | 15-20 weeks |
| Quad screen | 80-85% | 5-8% | ~5% | 15-20 weeks |
| Combined first-trimester (NT + PAPP-A + hCG) | 85-90% | 5% | ~5% | 11-13 weeks |
| Integrated (first + second trimester) | 90-95% | 4-5% | ~6% | Results at 16-18 weeks |
| NIPT (cell-free DNA) | > 99% | ＜ 0.5% | ~50-80% | 10+ weeks |
Sensitivity and Specificity Data
Understanding test performance requires looking at both sensitivity (detection rate) and specificity (correct identification of unaffected pregnancies).
| Screening Test | Sensitivity | Specificity | Positive Likelihood Ratio | Negative Likelihood Ratio |
|---|---|---|---|---|
| Triple screen | 0.67 (95% CI: 0.62-0.72) | 0.93 (95% CI: 0.92-0.94) | 9.6 | 0.36 |
| Quad screen | 0.82 (95% CI: 0.78-0.86) | 0.93 (95% CI: 0.92-0.94) | 11.7 | 0.19 |
| NIPT | 0.992 (95% CI: 0.985-0.997) | 0.998 (95% CI: 0.997-0.999) | 496 | 0.008 |
The positive likelihood ratio tells you how much to increase your baseline risk based on a positive result. NIPT's ratio of approximately 500 means a positive result increases your baseline risk 500-fold, making it far more informative than serum screening alone.
AFP Alone vs. Combined Screening
Using AFP alone to screen for Down syndrome is not recommended and is essentially never done in current clinical practice. AFP as a single marker has these limitations:
| AFP Alone Performance | Value |
|---|---|
| Detection rate | 30-35% |
| False positive rate | 10-15% |
| Overlap between affected and unaffected pregnancies | Substantial |
| Clinical utility as standalone test for Down syndrome | None |
AFP only contributes meaningfully when combined with other markers in the triple or quad screen. Its role is to modify the risk calculated from hCG, estriol, and age.
NIPT as a Better Alternative
Non-invasive prenatal testing (NIPT) analyzes cell-free fetal DNA fragments in the maternal bloodstream. Since its introduction in 2011, NIPT has progressively replaced second-trimester serum screening in many clinical settings.
NIPT Performance for Down Syndrome
| Metric | NIPT Value | Triple/Quad Screen Comparison |
|---|---|---|
| Detection rate | > 99% | 65-85% |
| False positive rate | ＜ 0.5% | 5-8% |
| Testing window | 10 weeks onward | 15-20 weeks |
| Time to results | 5-10 business days | 3-7 business days |
| Cost (US average) | $200-800 (insurance often covers) | $100-300 |
| Requires amniocentesis for confirmation | Yes (positive results only) | Yes (positive results only) |
| Detects other conditions | Trisomy 18, 13, sex chromosome aneuploidy | Limited |
When NIPT Is Preferred
ACOG recommends that all women be offered NIPT regardless of age or risk factors. NIPT is particularly recommended for:
- Women of advanced maternal age (35+)
- Women with abnormal ultrasound findings
- Women with a previous pregnancy affected by chromosomal abnormality
- Women who are carriers of balanced translocations
- Any patient who wants the most accurate screening available
When AFP Screening Is Still Used
Despite the superiority of NIPT, AFP-based screening remains relevant in certain situations:
- Neural tube defect screening: NIPT does not screen for neural tube defects; AFP remains the primary serum marker
- Cost or insurance barriers: Some insurance plans do not cover NIPT for low-risk patients
- Late presentation: Women who begin prenatal care after 13-14 weeks may have missed the optimal NIPT window (though NIPT can be done later)
- Patient preference: Some patients choose serum screening after counseling about options
- Resource-limited settings: NIPT may not be available in all clinical settings
What to Do with Screening Results
| Screening Result | Recommended Next Step |
|---|---|
| NIPT negative | No further testing for Down syndrome; routine prenatal care |
| NIPT positive | Confirmatory diagnostic testing (amniocentesis or CVS) with genetic counseling |
| Triple/quad screen negative | Routine prenatal care; NIPT can still be considered |
| Triple/quad screen positive | Genetic counseling; NIPT or direct diagnostic testing |
| Any screening positive + ultrasound findings | Diagnostic testing recommended; genetic counseling mandatory |
Diagnostic Testing Options
Screening tests estimate risk. Diagnostic tests confirm or rule out chromosomal abnormalities.
| Diagnostic Test | Timing | Procedure | Accuracy | Risk |
|---|---|---|---|---|
| Chorionic villus sampling (CVS) | 10-13 weeks | Needle through abdomen or cervix to sample placenta | > 99% | 0.5-1% miscarriage risk |
| Amniocentesis | 15-20 weeks | Needle through abdomen to sample amniotic fluid | > 99% | 0.1-0.3% miscarriage risk |
Both procedures provide fetal cells for karyotyping, microarray analysis, or other genetic testing. Results typically take 7-14 days for standard karyotype and 2-3 days for rapid FISH analysis.
Frequently Asked Questions
Does a low AFP mean my baby has Down syndrome?
No. A low AFP alone does not diagnose Down syndrome. AFP is only one component of a risk calculation that also includes hCG, estriol, maternal age, and sometimes inhibin A. Even with a "screen positive" result from the triple or quad screen, most babies do not have Down syndrome. Only diagnostic testing (CVS or amniocentesis) can confirm the diagnosis.
Is the quad screen better than NIPT?
NIPT is significantly more accurate than the quad screen for Down syndrome screening. NIPT detects over 99% of cases with a false positive rate below 0.5%, compared to the quad screen's 80-85% detection rate with a 5-8% false positive rate. However, the quad screen also screens for neural tube defects, which NIPT does not. Some patients choose to have both tests.
Can I skip the triple/quad screen and go straight to NIPT?
Yes. ACOG recommends that all women be offered NIPT regardless of age. If you choose NIPT, you can skip the triple or quad screen for chromosomal screening. However, you should still have the AFP component (or a detailed ultrasound) to screen for neural tube defects, as NIPT does not cover these conditions.
What is the false positive rate for Down syndrome screening?
The triple screen has a false positive rate of approximately 5-8%, meaning 5-8% of women screened will receive a "screen positive" result even though their babies do not have Down syndrome. The quad screen has a similar false positive rate but higher detection. NIPT has a false positive rate below 0.5%.
At what age should I get Down syndrome screening?
Current guidelines from ACOG recommend that all pregnant women be offered screening for chromosomal abnormalities regardless of age. The baseline risk increases with maternal age (1 in 1,200 at age 25 vs. 1 in 100 at age 40), but screening is valuable at any age because age alone is not a reliable predictor.
Key Takeaways
- AFP is low in Down syndrome pregnancies, not high — it is just one of several markers combined to estimate risk.
- The triple screen detects 65-70% of Down syndrome cases, while the quad screen detects 80-85% — both have 5-8% false positive rates.
- NIPT is the superior screening method, with > 99% detection and ＜ 0.5% false positive rate, and has become the preferred first-line test.
- AFP-based screening remains relevant for neural tube defect screening, which NIPT does not cover.
- All screening results must be confirmed with diagnostic testing (CVS or amniocentesis) before making clinical decisions.
- All pregnant women should be offered chromosomal screening regardless of age, per current ACOG guidelines.