How To Interpret Maternal Serum Afp Screening Results?

Maternal serum AFP screening is a blood test that measures alpha-fetoprotein, a protein made by the fetus, and reports it as a number compared to what is typical for your exact week of pregnancy. The result is never a diagnosis on its own. It is a risk estimate — a signal that tells your clinician whether further testing is worth discussing.

Higher-than-expected levels are associated with open neural tube defects, abdominal wall defects, and some placental issues. Lower-than-expected levels are associated with Down syndrome and a few other chromosomal conditions. Most abnormal results turn out to have a normal explanation, which is why the number matters less than what comes next.

What Is Maternal Serum AFP and What Does It Measure?

Alpha-fetoprotein is a protein produced first by the fetal yolk sac and later mainly by the fetal liver. Some of it crosses into the amniotic fluid and then into your bloodstream. That is why a sample of your blood can reflect something happening in the fetus.

AFP is measured between roughly 15 and 20 weeks of pregnancy for screening purposes. The result is reported in two ways. You may see it as a raw concentration in nanograms per milliliter (ng/mL), or as a multiple of the median (MoM). The MoM is the number that matters most for interpretation.

MoM means your value divided by the median value for pregnancies at the same gestational age. A result of 1.0 MoM means your level sits exactly at the middle of the reference group. A result of 2.5 MoM means your level is two and a half times that middle value.

AFP is also used in a different context entirely — monitoring certain liver and germ cell tumors in non-pregnant adults. That is a separate test with separate reference ranges and is not what this article covers.

What Do High AFP Results Mean?

A high AFP result means your level is above the expected range for your gestational age, usually reported as something like 2.0 or 2.5 MoM and above, depending on the laboratory and the specific cutoff used. That threshold is not universal, so the exact number that triggers follow-up varies by lab and clinic.

Elevated AFP is associated with several conditions:

• Open neural tube defects such as spina bifida and anencephaly. In these conditions, the open tissue allows more AFP to leak into the amniotic fluid and then into your blood.
• Abdominal wall defects such as gastroschisis and omphalocele, which work through a similar mechanism.
• Underestimated gestational age. If you are further along than the dating suggested, the expected AFP range would be higher and your value might actually be normal.
• Multiple pregnancy. Two fetuses produce more AFP than one.
• Placental abnormalities, including placental abruption or placental injury.
• Fetal demise, though this is usually apparent from other findings.

An important point that gets lost in the anxiety of an abnormal result: most elevated AFP values are not associated with any of these conditions. The test is designed to catch the cases that need a closer look, which means it also flags many pregnancies that are entirely normal. A high AFP is a reason to look further, not a reason to conclude anything.

What Do Low AFP Results Mean?

A low AFP result, generally below about 0.5 MoM, is associated with an increased risk of Down syndrome (trisomy 21) and, to a lesser degree, trisomy 18. The mechanism is not fully understood, but affected pregnancies tend to produce less AFP than expected.

Low AFP is rarely interpreted alone. It is usually combined with other markers — unconjugated estriol, hCG, and sometimes inhibin A — in what is called the quad screen or a related panel. The combined result gives a much better risk estimate than AFP by itself.

Low AFP is also seen in pregnancies with inaccurate dating. If you are earlier than estimated, your expected AFP would be lower, and a value that looks low might actually be normal for your true gestational age.

About one in every hundred pregnancies will have an abnormal AFP result of some kind. The vast majority of those pregnancies are unaffected by any of the conditions the test screens for.

How Is AFP Reported — MoM, Percentile, and Risk Ratios?

Different labs report AFP in different ways, and the format can be confusing. The table below explains what each format means.

The MoM is the most useful single number to focus on. It already accounts for gestational age, which is the biggest factor affecting AFP levels. A raw ng/mL value without a gestational age reference is essentially uninterpretable.

What Happens After an Abnormal Result?

An abnormal AFP result leads to a conversation, not a conclusion. Your clinician will typically start by confirming gestational age with an ultrasound, since dating errors are one of the most common explanations for an out-of-range result.

If dating is confirmed, a detailed ultrasound — sometimes called a targeted or level II ultrasound — is usually the next step. A skilled sonographer can often identify or rule out open neural tube defects and abdominal wall defects directly. In many cases, the ultrasound is normal and the AFP result turns out to have been a false positive.

If the ultrasound is inconclusive or the risk remains elevated, amniocentesis may be offered. This involves sampling amniotic fluid and measuring AFP and acetylcholinesterase directly. It is more accurate than the blood test but carries a small risk of miscarriage, which is why it is not done routinely. The exact risk varies by center and operator.

For low AFP results, the follow-up usually involves a more detailed risk assessment combining all the markers in the screening panel, and possibly cell-free DNA testing or diagnostic testing depending on the level of concern.

What Are the Limits of AFP Screening?

AFP screening is a risk assessment tool, not a diagnostic test. It cannot tell you whether your baby has a specific condition. It can only tell you whether the chance is high enough to justify further testing.

The test also has a meaningful false positive rate. Depending on the cutoff used, a substantial number of pregnancies flagged as high risk turn out to be unaffected. This is a known and accepted trade-off — the test is designed to miss as few true cases as possible, which means it will flag some normal pregnancies.

AFP screening does not detect all neural tube defects. Closed defects, where the tissue is not exposed to amniotic fluid, may not raise AFP at all. This is one reason ultrasound remains an important part of the evaluation.

The test is also not a substitute for the many other things that can affect a pregnancy. It screens for a specific set of conditions and nothing else. A normal AFP result does not guarantee a pregnancy without complications, and an abnormal result does not mean something is wrong.

What Factors Affect AFP Levels Besides Fetal Conditions?

Several things unrelated to fetal health can shift your AFP result. Gestational age is the biggest one. Because AFP rises steadily through the second trimester, even a few days of dating error can push a result above or below the expected range.

Maternal weight matters. AFP is distributed in the bloodstream, so people with higher body weight tend to have lower AFP concentrations for the same fetal production. Most labs adjust for this, but the adjustment is not perfect.

Diabetes, particularly insulin-dependent diabetes, is associated with lower AFP levels. Race and ethnicity also affect AFP distribution, and some labs apply adjustments. Smoking has been associated with higher AFP levels in some studies.

Multiple pregnancy raises AFP substantially, which is why twin pregnancies use different reference ranges. And the specific laboratory and assay used can produce slightly different values, which is why results from different labs are not always directly comparable.