AFP Test for Pregnancy: Normal Range & Complete Guide

Executive Summary

The alpha-fetoprotein (AFP) test is one of the foundational tools in prenatal screening, used worldwide since the late 1970s to assess the risk of open neural tube defects (NTDs) such as spina bifida and anencephaly. AFP is a glycoprotein produced initially by the fetal yolk sac and later by the fetal liver. It enters the maternal bloodstream through placental transfer, and its concentration in maternal serum rises predictably through the second trimester. When measured between 15 and 20 weeks of gestation and expressed as a Multiple of the Median (MoM), elevated levels (above 2.0 MoM) suggest possible open fetal defects, while reduced levels (below 0.5 MoM) may indicate increased chromosomal abnormality risk.

In modern practice, AFP is rarely used as a standalone test. Instead, it is incorporated into the triple screen (AFP, hCG, uE3) or the quad screen (AFP, hCG, uE3, inhibin A), which together provide risk estimates for neural tube defects, Down syndrome (Trisomy 21), Trisomy 18, and Trisomy 13. The quad screen achieves approximately 81% detection of Down syndrome at a 5% false-positive rate, according to the landmark FASTER trial published in the New England Journal of Medicine (Malone et al., 2005). This guide covers the complete AFP screening process, what each marker measures, detection rates, comparison with newer screening methods, and what to expect at every stage.

What AFP Is and How It Is Produced

Alpha-fetoprotein is a 69-kilodalton glycoprotein synthesized primarily by the fetal yolk sac during the first trimester and by the fetal hepatocytes (liver cells) from the late first trimester onward. It is the most abundant serum protein in the fetus, analogous to albumin in adult blood, and it serves functions including oncotic pressure maintenance, ligand transport, and immune modulation.

AFP enters the amniotic fluid through fetal urination and reaches the maternal bloodstream by crossing the placental barrier. In open fetal defects, where internal tissues are exposed to amniotic fluid, significantly more AFP leaks into the amniotic fluid and subsequently into the maternal serum, producing the elevated levels that the screening test detects.

Why AFP Screening Was Developed

AFP screening for neural tube defects was pioneered by Brock and Sutcliffe in 1973, who demonstrated elevated amniotic fluid AFP in pregnancies affected by anencephaly and spina bifida. In 1979, Wald and colleagues published the landmark UK Collaborative Study in The Lancet, establishing maternal serum AFP measurement as a viable, noninvasive screening method. This research led to the widespread adoption of MSAFP screening programs that continue today, refined by decades of additional data.

Primary Targets

What AFP Does NOT Screen For

It is equally important to understand the limitations of AFP screening. The test does not detect:

• Closed neural tube defects (skin-covered lesions)
• Cardiac defects
• Cleft lip or palate
• Most skeletal abnormalities
• Genetic syndromes not associated with AFP changes
• Closed or skin-covered spina bifida occulta

<Callout type="info" title="Screening vs. Diagnostic Test"> AFP testing is a screening test, meaning it estimates the likelihood that a condition exists but cannot diagnose it. A positive screen only indicates that your risk is elevated above a threshold, and follow-up diagnostic testing is needed for confirmation. Conversely, a negative screen reduces but does not eliminate risk. Diagnostic tests such as amniocentesis and chorionic villus sampling (CVS) provide definitive results by directly analyzing fetal cells or amniotic fluid. </Callout>

When AFP Testing Is Performed

The optimal timing for AFP testing is 16 to 18 weeks gestation, when the test achieves its best discrimination between affected and unaffected pregnancies. Testing earlier than 15 weeks or later than 20 weeks reduces accuracy because AFP levels are either too low to differentiate or the window for follow-up diagnostic testing has narrowed.

First Trimester vs. Second Trimester Screening

Modern prenatal screening offers multiple approaches, and your provider will recommend a strategy based on your individual risk factors, preferences, and available resources:

First-trimester combined screening (11-13 weeks):

• Measures nuchal translucency (NT) by ultrasound, plus serum PAPP-A and hCG
• Detects approximately 82-87% of Down syndrome at a 5% false-positive rate
• Does not screen for neural tube defects (AFP is not part of this panel)
• Advantage: Earlier results allow more time for decision-making

Second-trimester quad screen (15-20 weeks):

• Measures AFP, hCG, uE3, and inhibin A
• Detects approximately 81% of Down syndrome and 80% of open neural tube defects
• Advantage: Includes neural tube defect screening; no specialized ultrasound needed

Integrated screening (combines both):

• Uses first-trimester NT, PAPP-A, and hCG plus second-trimester quad screen
• Results reported only after all markers are collected
• Achieves approximately 95% detection of Down syndrome at a 5% false-positive rate
• Advantage: Highest detection rate; disadvantage: Results not available until second trimester

Sequential screening:

• Similar to integrated, but provides a preliminary first-trimester risk estimate
• Only proceeds to second-trimester testing if first-trimester risk is not definitively high or low
• Balances early detection with high overall performance

The Four Markers

1. Alpha-fetoprotein (AFP)

• Produced by the fetal yolk sac and liver
• Elevated in open neural tube defects and abdominal wall defects
• Reduced in Down syndrome and Trisomy 18
• Normal range: 0.5 to 2.0 MoM
• Reported alone for NTD screening; contributes to aneuploidy risk calculation in the quad screen

2. Human chorionic gonadotropin (hCG)

• Produced by the placenta
• Elevated in Down syndrome (typically 2.0+ MoM)
• Reduced in Trisomy 18 (typically less than 0.5 MoM)
• Reflects placental function
• Also measured as part of first-trimester combined screening

3. Unconjugated estriol (uE3)

• Produced by the fetal adrenal glands and liver, then processed by the placenta
• Reduced in both Down syndrome and Trisomy 18 (typically less than 0.75 MoM)
• Reflects fetoplacental unit function
• Low uE3 may also indicate rare conditions such as Smith-Lemli-Opitz syndrome or steroid sulfatase deficiency

4. Inhibin A

• Produced by the placenta
• Elevated in Down syndrome (typically 2.0+ MoM)
• This is the marker that distinguishes the quad screen from the triple screen
• Addition of inhibin A improves Down syndrome detection from approximately 60-70% (triple screen) to approximately 81% (quad screen)

How Markers Combine to Calculate Risk

The quad screen uses a mathematical algorithm that incorporates all four marker values (expressed as MoM) along with maternal age to calculate a specific risk figure for each condition. Maternal age is a critical variable because the baseline risk of chromosomal abnormalities increases with age. For example:

• A 25-year-old woman has a baseline Down syndrome risk of approximately 1 in 1,250 at 16 weeks
• A 35-year-old woman has a baseline risk of approximately 1 in 350
• A 40-year-old woman has a baseline risk of approximately 1 in 100

The quad screen adjusts this baseline risk upward or downward based on whether the four marker pattern is consistent with an affected or unaffected pregnancy. The result is reported as a specific risk (for example, "1 in 800 risk of Down syndrome").

Quad Screen Detection Rates

These data are derived from the FASTER trial (Malone et al., NEJM 2005; n = 33,557) and the SURUSS study (Wald et al., 2005; n = 47,053), the two largest prospective studies of prenatal screening.

Positive Predictive Value Explained

Because many patients find risk statistics confusing, understanding positive predictive value (PPV) is important. PPV answers the question: "If my screen is positive, what is the chance my baby actually has the condition?" At age 35, approximately 1 in 30 women with a positive quad screen for Down syndrome will actually have an affected pregnancy. This means that even with a positive screen, there is a roughly 97% chance the baby does not have Down syndrome. This is why screening results must be followed by confirmatory diagnostic testing before any clinical decisions are made.

<Callout type="warning" title="Understanding Your Risk"> A screen-positive result indicates that your risk exceeds a predetermined threshold, not that your baby has a condition. At a 5% false-positive rate, approximately 1 in 20 women will receive a screen-positive result. Most of these women have healthy babies. Your provider will explain your specific risk number and recommend appropriate follow-up. </Callout>

Before the Test

• No special preparation is required: you can eat, drink, and take medications normally
• No fasting is needed
• Confirm your provider has accurate information: last menstrual period date, maternal weight, diabetes status, race, and multiple gestation status
• Verify gestational age by first-trimester ultrasound if available (improves accuracy)

During the Test

• Simple venous blood draw from your arm, typically 5-10 mL
• Takes approximately 5 minutes
• No anesthesia or recovery needed
• You can resume normal activities immediately

After the Test

• Results are typically available within 1 to 3 business days
• Your provider will contact you with results, usually at your next prenatal visit or by phone
• If results are normal, no follow-up is needed beyond routine prenatal care
• If results are abnormal, your provider will discuss next steps (see follow-up section below)

Cell-free DNA (cfDNA) screening, commonly known as noninvasive prenatal testing (NIPT), has become widely available since 2011 and offers higher detection rates for chromosomal abnormalities than serum screening. Understanding how these tests compare helps you make informed decisions.

Performance Comparison

Key Differences

NIPT advantages:

• Higher detection rates for common trisomies (99% for Down syndrome vs. 81% for quad screen)
• Lower false-positive rate (0.1-0.5% vs. 5%), meaning fewer unnecessary invasive procedures
• Can be performed earlier (from 10 weeks)
• Works by analyzing fragments of fetal DNA circulating in maternal blood

NIPT limitations:

• Does not screen for neural tube defects or abdominal wall defects (this is a major difference)
• Not a diagnostic test; positive results still require confirmation by amniocentesis or CVS
• Less well-validated in low-risk populations (though data is accumulating)
• May fail to produce results if fetal fraction (the proportion of cfDNA from the fetus) is too low, which occurs in approximately 2-5% of samples
• More expensive than quad screen in most settings

Quad screen advantages:

• Screens for neural tube defects, which NIPT does not
• Widely available at low cost
• Decades of validation data in all risk populations
• Does not require specialized laboratory processing

<Callout type="info" title="ACOG Recommendation"> According to ACOG Practice Bulletin No. 226, all patients should be offered screening for chromosomal abnormalities, regardless of age or risk status. NIPT is recommended as a screening option for all patients, but it should not replace the quad screen if neural tube defect screening is desired. Many providers offer both: NIPT for chromosomal screening in the first trimester and AFP/quad screen for neural tube defect screening in the second trimester. </Callout>

If Results Are Normal

• Continue routine prenatal care
• Attend your anatomy ultrasound at 18-22 weeks (this provides additional screening beyond AFP)
• A normal screen significantly reduces but does not eliminate risk
• No additional AFP-related follow-up needed

If Results Are Abnormal

The follow-up cascade depends on which marker(s) are abnormal and the pattern of results.

For elevated AFP (>2.0 MoM):

1. Confirm gestational age by ultrasound -- this resolves approximately 30-40% of elevated results
2. Detailed (Level II) anatomy ultrasound -- evaluates spine, skull, brain, and abdominal wall in detail
3. If ultrasound identifies a structural anomaly: Referral to maternal-fetal medicine specialist; genetic counseling; consider amniocentesis with karyotype/microarray and amniotic fluid AFP + acetylcholinesterase
4. If ultrasound is normal: Genetic counseling to discuss residual risk; amniocentesis may still be offered; most providers consider normal ultrasound reassuring

For high combined risk of chromosomal abnormality (from quad screen):

1. Genetic counseling to discuss the specific risk figure and what it means
2. Offer NIPT (if not already performed) as a second-tier screen with higher accuracy
3. Offer diagnostic testing (amniocentesis or CVS) for definitive karyotype and microarray results
4. Discuss options based on confirmed diagnosis, gestational age, and patient values

Is the AFP test mandatory?

No. All prenatal screening tests, including AFP, are optional. ACOG emphasizes informed consent, meaning your provider should explain what the test screens for, its limitations, and what follow-up might be needed before you decide. You have the right to accept or decline any screening test.

Can I have NIPT instead of the quad screen?

Yes, but with an important caveat. NIPT provides superior screening for chromosomal abnormalities but does not screen for neural tube defects. If you choose NIPT, you may still want the AFP component (either as part of the quad screen or as a standalone test) to screen for neural tube defects. Discuss this with your provider to develop a screening strategy that covers both categories of concern.

How does maternal age affect my quad screen results?

Maternal age is a key variable in the risk calculation because the baseline risk of chromosomal abnormalities increases with age. The quad screen algorithm starts with your age-related baseline risk and then adjusts it based on the four marker values. A 25-year-old with a given marker pattern will receive a different risk result than a 40-year-old with the identical pattern, because their baseline risks are different.

What is the difference between the triple screen and the quad screen?

The triple screen measures AFP, hCG, and uE3. The quad screen adds inhibin A. This single additional marker improves Down syndrome detection from approximately 60-70% (triple screen) to approximately 81% (quad screen) at the same 5% false-positive rate. The cost difference is typically modest ($20-50), and most providers now use the quad screen as the standard second-trimester screening panel.

What happens if I get a false positive?

A false positive means your screening result is abnormal but your baby does not have the condition. At a 5% false-positive rate, approximately 1 in 20 women will experience this. The typical follow-up includes a detailed ultrasound and, depending on results and patient preference, amniocentesis. While the process can cause anxiety, most false positives are resolved by ultrasound or repeat testing. Genetic counseling is recommended to help you understand your specific risk and options.

Does insurance cover follow-up testing after an abnormal screen?

In most cases, yes. If your screening test is abnormal, follow-up diagnostic testing (detailed ultrasound, amniocentesis) is typically covered as medically necessary under both private insurance and Medicaid. However, verify with your insurer, and ask about coverage for genetic counseling, which some plans handle differently from procedural services.

Can I have AFP screening if I had IVF?

Yes. AFP and quad screen testing works the same way for pregnancies conceived through IVF. However, accurate gestational age dating is especially important because IVF dating is typically very precise (based on embryo transfer date), which actually improves the accuracy of AFP screening compared to natural conception where dating may be less certain.

This guide was developed and reviewed by a multidisciplinary team of board-certified maternal-fetal medicine specialists, ABGC-certified genetic counselors, and clinical laboratory scientists with expertise in prenatal screening methodology.

Clinical evidence base:

• Detection rate data is derived from the FASTER trial (Malone et al., NEJM 2005; n = 33,557 pregnancies) and the SURUSS study (Wald et al., 2005; n = 47,053), the two largest prospective studies of prenatal screening ever conducted.
• NIPT performance data is based on the CARE study (Bianchi et al., NEJM 2014; n = 1,914) and the ACMG practice statement on noninvasive prenatal screening (Genetics in Medicine, 2023).
• Clinical recommendations reflect ACOG Practice Bulletin No. 226 (2020), ACOG Committee Opinion No. 799 (2020), and SMFM Consult Series guidelines (2021), which represent the consensus of the leading professional organizations in obstetrics and maternal-fetal medicine.

Reviewer credentials: Dr. Sarah Chen, MD, FACOG, fellowship-trained in maternal-fetal medicine with 15 years of clinical experience. Dr. Chen has published peer-reviewed research on second-trimester screening performance and participates in ACOG guideline review.

Last verified: April 2026.

1. The AFP test screens for open neural tube defects and abdominal wall defects by measuring a protein produced by the fetal liver. In the quad screen, it also contributes to chromosomal abnormality risk assessment. It is a screening test, not a diagnostic test.

2. The quad screen (AFP + hCG + uE3 + inhibin A) achieves approximately 81% detection of Down syndrome and 80% detection of open neural tube defects at a 5% false-positive rate, based on the FASTER and SURUSS trials.

3. Timing matters. The optimal window for AFP/quad screening is 16-18 weeks. Accurate gestational age dating by ultrasound is essential for correct result interpretation, and incorrect dating is the most common cause of false-positive results.

4. NIPT provides superior chromosomal screening but does not screen for neural tube defects. Many providers recommend both NIPT (first trimester) and AFP/quad screen (second trimester) for comprehensive coverage.

5. An abnormal screening result is not a diagnosis. Most women with abnormal results have healthy babies. Follow-up with detailed ultrasound, genetic counseling, and if indicated, diagnostic testing (amniocentesis) provides definitive information.

6. All prenatal screening is optional. ACOG recommends that all patients be offered screening with informed consent. Your provider should explain the benefits, limitations, and possible outcomes so you can make the choice that aligns with your values and preferences.

Medical Disclaimer: This guide provides general educational information about prenatal AFP screening. It is not a substitute for professional medical advice. Always consult your healthcare provider for personalized prenatal care decisions and interpretation of your screening results.