AFP, or alpha-fetoprotein, is a protein produced by your developing baby that crosses the placenta into your bloodstream. Measuring its level through a simple blood draw between weeks 15 and 22 of pregnancy helps screen for neural tube defects like spina bifida, abdominal wall defects, and chromosomal conditions like Down syndrome. It’s one of the most established prenatal screening tools, and understanding what it measures, what the results mean, and what happens next can take a lot of the anxiety out of the process.
Where AFP Comes From
In early pregnancy, the yolk sac produces AFP. As the fetus develops, the fetal liver takes over as the primary source, with smaller amounts coming from the fetal gastrointestinal tract. AFP also shows up in the baby’s cerebrospinal fluid and in the amniotic fluid (filtered there through the baby’s kidneys).
The placenta itself does not produce AFP, but it plays a key role in getting the protein into your blood. During the first trimester, AFP passes through the fetal membranes. Starting in the second trimester, specialized receptors on the placenta allow direct transport into maternal circulation. That’s why the test is timed for the second trimester: there’s enough AFP crossing the placenta by then to give a meaningful reading.
When the Test Is Done
The blood draw can happen any time between 15 and 22 weeks of pregnancy, counted from the first day of your last menstrual period. The sweet spot is weeks 16 through 18, when the measurement is most accurate. Getting the gestational age right matters a lot here, because AFP levels change week by week. If your due date is off by even a week or two, the result can look falsely high or low.
AFP as Part of the Quad Screen
AFP is rarely interpreted on its own. Most providers order it as part of the quad screen, which measures four substances in your blood:
- Alpha-fetoprotein (AFP): the protein made by the baby
- Human chorionic gonadotropin (hCG): a hormone from the placenta
- Unconjugated estriol: a form of estrogen made by both the baby and the placenta
- Inhibin A: another placental hormone
When only the first three are measured, it’s called a triple screen. Adding inhibin A improves the accuracy of screening for Down syndrome. Looking at the pattern across all four markers gives a much clearer picture than any single number. For example, a specific combination of low AFP, low estriol, high hCG, and high inhibin A raises the probability of Down syndrome or Edwards syndrome, while high AFP alone points toward a different set of concerns.
What High AFP Levels Can Mean
An AFP level above the expected range is most commonly associated with neural tube defects, which are openings in the baby’s brain or spinal cord. The two main types are anencephaly (where parts of the brain and skull don’t form) and spina bifida (where the spinal column doesn’t close completely). When these defects are “open,” meaning the tissue is exposed rather than covered by skin, AFP leaks from the baby into the amniotic fluid and then into your bloodstream in higher than normal amounts.
High AFP is also linked to abdominal wall defects like gastroschisis and omphalocele, where the baby’s intestines or other organs develop outside the body. Less common associations include kidney abnormalities, certain rare tumors, placental problems, and a brittle bone condition called osteogenesis imperfecta. In some cases, elevated AFP simply reflects a threatened miscarriage or lower than expected fetal growth rather than a structural problem.
What Low AFP Levels Can Mean
When AFP comes back lower than expected, the concern shifts to chromosomal conditions. Low AFP is one piece of the pattern seen in pregnancies affected by Down syndrome (trisomy 21) or Edwards syndrome (trisomy 18). On its own, a low AFP number doesn’t diagnose anything. It’s the combination with the other quad screen markers that determines whether further testing is recommended.
How Results Are Reported
Your result won’t be given as a simple number in the way a cholesterol test might be. Instead, labs convert it to “multiples of the median,” or MoM. This compares your AFP level to the median level for all pregnancies at the same gestational age. A MoM of 1.0 means your level is right at the middle. For singleton pregnancies, a common cutoff for flagging a high result is 2.5 MoM.
The MoM calculation also adjusts for factors that naturally shift AFP levels. Your weight matters because a heavier person has more blood volume, which dilutes the AFP concentration. Race and ethnicity affect baseline levels as well. Preexisting diabetes has historically been thought to lower AFP, leading some labs to apply a correction factor, though more recent research suggests that once weight is accounted for, the diabetes adjustment may not be necessary.
False Positives and Common Mix-Ups
A screening test is not a diagnosis, and AFP screening has a notable false positive rate. Among singleton pregnancies tested at 16 to 18 weeks, roughly 3 percent of results come back above the 2.5 MoM threshold without any actual problem. The most frequent explanation is simply inaccurate dating: if you’re a week or two earlier than estimated, your AFP will be lower than expected for the assumed gestational age, and vice versa.
Twin pregnancies are another major source of false alarms. Two babies produce roughly twice the AFP, so applying the standard singleton cutoff flags about 30 percent of unaffected twin pregnancies as abnormal. Labs use a higher cutoff (around 5.0 MoM) for twins to bring the false positive rate back down to a more useful range, though this comes at the cost of catching fewer true cases of spina bifida.
What Happens After an Abnormal Result
If your AFP screen comes back outside the normal range, the first step is almost always an ultrasound. A detailed scan can confirm your due date (correcting the most common cause of a false result), check whether you’re carrying multiples, and look directly at the baby’s spine, skull, and abdominal wall. Many abnormal AFP results are resolved at this stage with no further testing needed.
If the ultrasound doesn’t explain the result, the next option is amniocentesis, a procedure where a small sample of amniotic fluid is drawn through a thin needle. The AFP level in amniotic fluid is far more specific than the blood test. The fluid can also be analyzed for chromosomal conditions, providing a definitive answer rather than a probability estimate. Amniocentesis is typically offered between weeks 15 and 20, which aligns with the same window as the blood screening.
It’s worth keeping in mind that the vast majority of pregnancies flagged by AFP screening ultimately turn out to be healthy. The test is designed to cast a wide net so that serious conditions aren’t missed, which means many people will go through the follow-up process and receive reassuring news at the end of it.