Pregnancy is filled with new experiences—and often, lots of medical tests. Among them, a routine blood test called the AFP test may show up in your second trimester. If you’re wondering what AFP stands for or why your doctor is interested in your protein levels, you’re not alone.
AFP, or alpha-fetoprotein, is a protein produced by your developing baby that can offer important clues about their health. While it might sound technical or even intimidating, this screening test plays a valuable role in assessing your baby’s risk for certain birth defects and genetic conditions.
In this blog, we’ll break down what AFP levels mean, how they’re measured, and what they can (and can’t) reveal about your baby’s development, so you can feel more informed and empowered at your next prenatal appointment.
To help explain what the AFP test is, why it matters, and what the results may mean, we consulted Dr. Michael D. Wittenberger, a Board Certified Reproductive Endocrinologist at HRC Fertility. With over 15 years of experience treating infertility, performing restorative surgeries, and guiding patients through fertility preservation, Dr. Wittenberger is known for his compassionate, collaborative approach to care.
What is AFP (alpha-fetoprotein)?
Alpha-fetoprotein (AFP) is a protein made by your baby’s liver and yolk sac during early development. It plays a role similar to albumin (a major protein in adult blood), helping transport substances like hormones and fatty acids in the fetal bloodstream.
Normally, AFP is present in amniotic fluid and crosses into your bloodstream through the placenta. That’s why a simple blood draw from you can give insight into your baby’s health.
“ Alpha-fetoprotein is a protein synthesized by the fetal liver, intestinal tract and yolk sac. Its exact function in the fetus is unknown. However, it is also found in the maternal serum (blood) and its level can be correlated with certain pregnancy risks like open-neural tubes, abdominal wall defects, fetal urinary obstruction, fetal anemia and some fetal tumors.”
AFP levels naturally change throughout pregnancy, rising until around 32 weeks and then declining. Measuring these levels during a specific window in the second trimester allows doctors to screen for certain conditions related to the baby’s brain, spine, and chromosomes.
According to the National Library of Medicine, AFP is the most abundant plasma protein in the human fetus and is widely used as a biomarker in maternal serum screening.
When is the AFP test done during pregnancy?
AFP testing is typically done between 15 and 20 weeks of pregnancy, with the most accurate results falling in the 16–18 week window. It’s often included in the second-trimester maternal serum screening, also known as the quad screen, which evaluates four substances in your blood:
- Alpha-fetoprotein (AFP)
- Human chorionic gonadotropin (hCG)
- Unconjugated estriol
- Inhibin A
According to Dr. Wittenberger, standardized levels called multiples of the mean (MoM) are available for these gestational ages and remain consistent between various laboratories, allowing for accurate diagnosis regardless of where the patient is tested.
“Earlier testing of AFP is not recommended as it has poor sensitivity for detecting these conditions. Later testing may limit the patient’s options for some interventions,” he says.
It’s important to note: AFP testing is a screening tool, not a diagnostic test. An abnormal result doesn’t mean something is definitely wrong—it simply means that further testing might be needed to clarify the picture.
How is the AFP test performed?
The AFP test is a simple maternal blood draw, usually performed during a routine prenatal visit between 15 and 20 weeks of pregnancy. Once collected, your blood is sent to a lab, where the level of alpha-fetoprotein is measured and reported as a value called “MoM,” or multiples of the median.
MoM compares your AFP level to the average level for a pregnant person at the same gestational age. For example:
- 1.0 MoM = average
- Above 2.5 MoM = considered elevated
- Below 0.5 MoM = considered low
Adjustments are made based on factors like your gestational age, weight, race, diabetes status, and whether you’re carrying multiples—all of which can influence AFP levels. This helps reduce false positives and makes interpretation more accurate.
What do high AFP levels mean?
High levels of AFP in maternal blood, typically above 2.5 MoM, can be a red flag that warrants further investigation. Elevated AFP may signal an increased risk for certain conditions, particularly involving the neural tube and fetal development.
Accroding to Dr. Wittenberger, “Levels of AFP are dependent on the gestational age and maternal weight at the time the blood was drawn. Maternal diseases like diabetes can also affect serum AFP levels and need to be noted.”
He further says that elevated AFP levels may also indicate that there is more than one fetus present. Fetal death, as seen with a vanishing twin, can also result in elevated maternal serum AFP levels.
Here are some possible reasons for high AFP levels:
- Neural tube defects (NTDs)
Conditions like spina bifida or anencephaly occur when the fetal spinal cord or brain doesn’t develop properly. These defects allow AFP to leak into the amniotic fluid and maternal bloodstream, leading to elevated levels. A large-scale study published in Acta Obstetricia et Gynecologica Scandinavica found a strong association between high maternal AFP levels and open neural tube defects, especially when measured between 16 and 18 weeks of gestation.
- Abdominal wall defects
Conditions like gastroschisis or omphalocele, where the baby’s intestines or other organs develop outside the body, can also cause AFP to rise because of direct exposure to amniotic fluid.
- Underestimated gestational age
If your pregnancy is further along than estimated, AFP levels will naturally be higher. An ultrasound may be needed to confirm gestational dating.
- Multiple pregnancies
Carrying twins or triplets can raise AFP levels, since each fetus produces AFP. This is usually taken into account when interpreting test results.
- Placental problems
In rare cases, issues like placental abruption or tumors can cause elevated AFP. However, these findings typically require follow-up with ultrasound or additional maternal-fetal testing.
It’s important to remember: Most people with high AFP results go on to have healthy pregnancies. The test only estimates risk—it doesn’t confirm a diagnosis.
What do low AFP levels mean?
While high AFP levels are often linked to structural abnormalities, low AFP levels—typically below 0.5 MoM—can indicate an increased risk of certain chromosomal conditions, especially when considered alongside other markers in the quad screen.
Here are the main concerns associated with low AFP:
- Down syndrome (trisomy 21)
Low maternal AFP levels are one of the key indicators in screening for Down syndrome. When combined with high hCG and low estriol levels, the risk assessment becomes more significant. A study published in the New England Journal of Medicine found that second-trimester AFP levels, when used as part of a multi-marker screen, significantly improved early detection rates for Down syndrome.
- Trisomy 18 (Edwards syndrome)
This rare and severe condition is also associated with low AFP, along with low levels of hCG and estriol. Trisomy 18 is often incompatible with life and may be flagged during routine screening before being confirmed by diagnostic testing like amniocentesis.
- Overestimated gestational age
If your pregnancy is earlier than estimated, your AFP levels may appear artificially low. A dating ultrasound can help adjust timelines and reduce confusion.
It’s essential to remember: AFP is just one piece of a much larger puzzle. Many pregnancies with abnormal AFP levels turn out completely healthy, especially when no other risk factors are present.
How accurate is the AFP test?
AFP screening is a valuable risk assessment tool, but it’s far from definitive. While it can highlight potential concerns, it cannot diagnose specific conditions. Its accuracy depends on several variables, including correct gestational dating, maternal weight, and whether it’s part of a multi-marker screen like the quad test.
Sensitivity and specificity
AFP screening has moderate sensitivity for detecting neural tube defects. When combined with other markers (as in the quad screen), its ability to identify Down syndrome improves. According to a study in the American Journal of Obstetrics and Gynecology, the quad screen has a detection rate of about 75% for Down syndrome and over 85% for open neural tube defects, with a false positive rate of 5%.
Why false positives and negatives happen
AFP levels can be influenced by factors unrelated to fetal health, such as:
- Incorrect estimation of gestational age
- Maternal weight and race
- Multiple gestation
- Certain maternal health conditions
These confounding factors can lead to false positives, causing unnecessary stress—or false negatives, which may give a false sense of reassurance.
What happens after an abnormal result?
If your AFP level falls outside the typical range, your provider may recommend:
- A targeted ultrasound to look for physical anomalies
- Noninvasive prenatal testing (NIPT) for chromosomal screening
- Amniocentesis for a diagnostic look at your baby’s genetic material
In short, AFP screening helps flag pregnancies that need a closer look—but doesn’t provide a final answer on its own.
Should you get the AFP test?
AFP testing (often as part of the quad screen) is recommended for all pregnant individuals who begin prenatal care in the second trimester, especially those who didn’t receive first-trimester screening. It’s a noninvasive, accessible tool that can offer valuable insight into fetal development and possible complications.
“If AFP levels are abnormal, an ultrasound is usually the next test performed to confirm gestational age, fetal number, fetal viability (i.e. fetal death) and whether there are any abnormalities present in the fetus to explain the elevated maternal serum AFP levels,” says Dr. Wittenberger.
The American College of Obstetricians and Gynecologists (ACOG) advises that all pregnant people should be offered prenatal genetic screening, including maternal serum screening like AFP, regardless of age or risk factors.
That said, whether or not to proceed with AFP testing is a personal decision. You might consider it if:
- You want more information about your baby’s development.
- You’re at higher risk for neural tube defects or chromosomal abnormalities.
- You missed first-trimester screening (like nuchal translucency or blood tests).
- You’re looking for noninvasive options before deciding on diagnostic tests.
On the other hand, some people may choose to skip AFP screening if:
- They’ve already done comprehensive first-trimester screening.
- They prefer not to know risk probabilities without definitive answers.
- They would not pursue further testing or interventions, regardless of the results.
Your provider can help weigh the pros and cons based on your medical history, family background, and emotional readiness for further testing.
Conclusion
AFP testing is a powerful early window into your baby’s development—but it’s not a diagnosis. Whether high or low, abnormal levels are simply a signal to look more closely. Most of the time, further testing like ultrasounds or genetic screening offers clarity—and often, reassurance. Talk to your provider about what your AFP results mean in the context of your full pregnancy picture. With the right follow-up, you’ll be better equipped to make informed, confident choices for you and your baby.
