AFP, or alpha-fetoprotein, is a protein produced by your developing baby’s liver and yolk sac during pregnancy. A simple blood test measures how much of this protein has crossed into your bloodstream, and the result helps screen for certain birth defects and genetic conditions. It’s one of the most common prenatal screening tests, typically offered in the second trimester.
What AFP Actually Is
As your baby grows, its liver and yolk sac produce alpha-fetoprotein in increasing amounts. Some of this protein passes through the placenta and into your blood. The amount circulating in your bloodstream changes throughout pregnancy, rising steadily and peaking around weeks 30 to 32 before declining. Because AFP levels follow a predictable pattern tied to gestational age, measuring the amount in your blood at a specific point in pregnancy can reveal whether levels are unusually high or low, both of which can signal different problems.
When and How the Test Is Done
The AFP blood test is drawn between 15 and 22 weeks of pregnancy, with 16 to 18 weeks considered the ideal window. It’s a standard blood draw from your arm, and results typically come back within one to two weeks. The test is a screening tool, not a diagnostic one. That distinction matters: a screening test tells you whether the risk of a problem is higher or lower than average, but it cannot confirm that a problem exists.
AFP can be measured on its own (sometimes called MSAFP, for maternal serum alpha-fetoprotein), but it’s more commonly part of a larger panel. The quad screen measures four substances in your blood at once: AFP, human chorionic gonadotropin (hCG, a hormone from the placenta), unconjugated estriol (a form of estrogen produced by the baby and placenta), and inhibin A (another placental hormone). Combining all four gives a more accurate risk estimate than AFP alone.
What High AFP Levels Can Mean
Higher-than-expected AFP in your blood raises the possibility of neural tube defects, which are openings in the baby’s brain or spinal cord that occur very early in development. The two most common types are spina bifida, where the spinal column doesn’t close completely, and anencephaly, where major parts of the brain and skull don’t form. AFP screening identifies roughly 80% of open spina bifida cases and about 95% of anencephaly cases as early as 16 weeks.
High AFP can also point to abdominal wall defects, where the baby’s intestines or other organs develop outside the body. These conditions cause elevated AFP because the protein leaks more freely into the amniotic fluid and then into your bloodstream when the baby’s skin or tissue covering is incomplete.
That said, a high result doesn’t necessarily mean something is wrong. Several non-threatening factors can push AFP levels up. Carrying twins or multiples is a common cause, since more than one baby means more AFP production. Incorrect gestational dating is another frequent explanation: if you’re further along than estimated, your AFP will naturally be higher than expected for the presumed week. For this reason, an ultrasound to confirm your due date is often the first follow-up step after an elevated result.
What Low AFP Levels Can Mean
Lower-than-normal AFP levels are associated with an increased risk of chromosomal conditions, particularly Down syndrome (trisomy 21) and Edwards syndrome (trisomy 18). Down syndrome affects how the baby’s body and brain develop, causing a range of intellectual and physical challenges that vary widely from person to person. Edwards syndrome is far less common and more severe, involving abnormalities in multiple organs; most babies with this condition do not survive their first year.
Low AFP alone is not enough to calculate risk for these conditions accurately. That’s why providers rely on the quad screen rather than a standalone AFP result when evaluating chromosomal risk. The four markers together, combined with your age and other factors, generate a more reliable risk number.
How Results Are Reported
AFP results are reported as “multiples of the median,” or MoM. The median is the middle value for all pregnancies at the same gestational age. A result of 1.0 MoM means your level is right at the median. Results significantly above or below 1.0 trigger further evaluation. Your provider will interpret the number in context, factoring in your weight, race, whether you have diabetes, and how many babies you’re carrying, since all of these influence what’s considered normal for you.
What Happens After an Abnormal Result
If your AFP result falls outside the expected range, the next step is almost always a detailed ultrasound. This can confirm your due date, check for twins, and look directly at the baby’s spine, skull, and abdominal wall. Many abnormal AFP results are explained by something straightforward, like a dating error, and the ultrasound clears things up without further testing.
If the ultrasound doesn’t explain the result or if it reveals a concern, your provider may recommend amniocentesis. This procedure takes a small sample of amniotic fluid, which contains the baby’s cells and its own AFP. Measuring AFP directly in the amniotic fluid is far more accurate than the blood screening test, and analyzing the baby’s chromosomes from the fluid can confirm or rule out genetic conditions like Down syndrome or Edwards syndrome. Amniocentesis is diagnostic, meaning it gives a definitive answer rather than a risk estimate.
AFP Screening vs. Cell-Free DNA Testing
If you’ve had cell-free DNA testing (also called NIPT, or non-invasive prenatal testing), you might wonder whether AFP screening is still necessary. NIPT is very accurate for detecting chromosomal conditions like Down syndrome and Edwards syndrome, but it does not screen for neural tube defects or abdominal wall defects. AFP remains the primary blood screening tool for those structural problems. Many providers recommend AFP screening even after a normal NIPT result, specifically to catch conditions that NIPT wasn’t designed to detect.
Some practices pair NIPT in the first trimester with a standalone AFP test in the second trimester, skipping the full quad screen. Others still offer the quad screen as a single comprehensive option. The right approach depends on your individual risk factors and what’s available through your provider.
Limitations Worth Understanding
AFP screening has a relatively high false-positive rate. Most people who get an abnormal result will go on to have a healthy pregnancy after follow-up testing. The screening catches real problems a meaningful percentage of the time, but the majority of flagged results turn out to be nothing. Understanding this upfront can help reduce anxiety if your number comes back outside the expected range. An abnormal screening result is the beginning of a conversation, not a diagnosis.