Maternal serum alpha-fetoprotein, usually abbreviated MSAFP, is a blood test offered during the second trimester of pregnancy that measures the concentration of alpha-fetoprotein (AFP) in the mother’s bloodstream. AFP is a protein produced by the developing fetus, primarily in the liver, and small amounts cross the placenta into maternal circulation. Abnormal levels can signal conditions ranging from open neural tube defects to chromosomal abnormalities, though the test is a screening tool rather than a diagnosis. For roughly four decades MSAFP was the frontline screen for spina bifida and anencephaly, though its role has shifted considerably with the rise of high-resolution ultrasound and cell-free DNA testing.
How Alpha-Fetoprotein Reaches the Mother’s Blood
AFP is produced in large quantities by the fetal liver and yolk sac during early development. It circulates in fetal blood, passes into amniotic fluid (mostly through fetal urine), and eventually crosses the placenta into the mother’s bloodstream. Research using perfused human placentas has shown that this transfer is not random leakage. AFP clearance from the fetal side to the maternal side was roughly 45 times greater than clearance in the reverse direction, meaning the placenta functions almost like a one-way valve for this protein. The transfer appears to occur through two routes: across small gaps in the outer layer of placental villi, and through vessels that pass through the base of the placenta where it contacts the uterine wall.1The Journal of Clinical Investigation. Mechanisms of alphafetoprotein transfer in the perfused human placental cotyledon from uncomplicated pregnancy
This directional flow matters clinically. Because the placenta actively channels AFP from fetus to mother, maternal AFP levels reflect what is happening on the fetal side. When fetal skin or neural tissue is open and exposed to amniotic fluid, more AFP leaks out, raising amniotic fluid levels and, in turn, maternal blood levels. Conversely, when the fetus produces less AFP than expected, maternal levels drop.
When the Test Is Done
MSAFP testing is performed during a specific window in the second trimester. The preferred timing is between 15 and 18 weeks of gestation, when sensitivity for detecting open neural tube defects is highest.2PubMed. Elevated Maternal Serum AFP (MSAFP) During the Second Trimester Testing can technically be done up to about 21 weeks, but accuracy declines at the edges of the window.3Genetics in Medicine. Second trimester maternal serum screening for fetal open neural tube defects and aneuploidy Gestational age at the time the blood draw is taken is critical to interpretation, and labs need to know it to generate an accurate result.
The reason timing matters so much is that AFP levels in maternal blood naturally change throughout pregnancy. They rise steadily during the second trimester, so a value that looks normal at 16 weeks might look abnormally low at 18 weeks if the lab does not know the true gestational age. This is one of the most common sources of misleading results.
How Results Are Reported
Rather than a simple “normal” or “abnormal” number, MSAFP results are reported as multiples of the median, or MoM. The lab takes the raw AFP concentration from your blood, compares it to the median value expected for your gestational age, and expresses your result as a ratio. A result of 1.0 MoM means your level is right at the median. A result of 2.0 MoM means yours is twice the median.
The MoM system exists because raw AFP values vary by gestational week, maternal weight, race, and whether the mother has diabetes. Converting to MoM after adjusting for these factors allows a single threshold to be applied across different populations. For neural tube defect screening, a threshold of 2.5 MoM is commonly used to flag pregnancies for further evaluation.4PubMed Central. All MoMs are not equal: some statistical properties associated with reporting results in the form of multiples of the median This does not mean that every pregnancy above 2.5 MoM has a neural tube defect. It means the probability is high enough to warrant follow-up testing.
For mothers with pregestational diabetes, labs apply a correction factor because diabetes can affect AFP metabolism. After proper adjustment, MSAFP levels in women with diabetes, whether on insulin or oral medications, are essentially the same as in non-diabetic women.5PubMed. Adjustment of maternal serum alpha-fetoprotein levels in women with pregestational diabetes
What a High MSAFP Can Indicate
The classic association with elevated MSAFP is an open neural tube defect, specifically spina bifida (where the spinal column does not close completely) or anencephaly (where large portions of the brain and skull fail to develop). In these conditions, fetal neural tissue is exposed to amniotic fluid, allowing large amounts of AFP to leak out and eventually reach the maternal bloodstream. For four decades, MSAFP was considered the gold standard screening method for these defects.6PubMed Central. Prenatal screening for neural tube defects: from maternal serum alpha-fetoprotein to ultrasonography
But neural tube defects are not the only reason MSAFP can be elevated. High levels have also been linked to a range of fetal and placental conditions, including severe fetal kidney disease (where increased protein loss into amniotic fluid raises AFP), fetal liver abnormalities that cause overproduction, gastrointestinal defects where the bowel is exposed to amniotic fluid, skin conditions that allow AFP to leak through blood vessels, and fetal anemia accompanied by placental swelling.7PubMed Central. Fetal and Placental Causes of Elevated Serum Alpha-Fetoprotein Levels in Pregnant Women
Common Reasons for Falsely Elevated Results
A significant number of pregnancies flagged with high MSAFP turn out, after further testing, to have no identifiable fetal abnormality. The most frequent culprit is incorrect dating. If the pregnancy is further along than estimated, the AFP level will be higher than expected for the assumed gestational age, producing a false positive. Multiple gestations, such as twins, also cause elevated MSAFP because two fetuses produce more AFP than one. These are not exotic scenarios. They represent a substantial portion of the “abnormal” results that prompt follow-up ultrasounds and, in some cases, amniocentesis.
This is why an accurate ultrasound to confirm gestational age is often the first step after an elevated MSAFP result. If redating the pregnancy brings the MoM value below the threshold, no further workup is needed.
What a Low MSAFP Can Signal
Low MSAFP levels are associated with an increased risk of chromosomal conditions, particularly Down syndrome (trisomy 21) and Edwards syndrome (trisomy 18). In pregnancies affected by Down syndrome, MSAFP tends to run below the population median, though the overlap with normal pregnancies is large enough that MSAFP alone is not a reliable screen.8PubMed. First-trimester alpha-fetoprotein screening for Down syndrome In trisomy 18, MSAFP levels are typically even lower than in Down syndrome pregnancies.
Early efforts to use low MSAFP as a standalone Down syndrome screen showed limited usefulness. In one prospective screening program, about 7% of women who underwent amniocentesis because of low MSAFP were found to be carrying a fetus with a chromosomal trisomy.9Pediatric Research. PROSPECTIVE MSAFP SCREENING FOR DOWN SYNDROME: THE BAYLOR EXPERIENCE That detection rate was better than nothing, but it also meant that the vast majority of women flagged for low MSAFP did not have an affected pregnancy. This is part of why MSAFP was eventually combined with other blood markers and, more recently, supplemented or replaced by newer technologies.
MSAFP as Part of the Quad Screen
In current practice, MSAFP is rarely interpreted in isolation. It is one component of a multi-marker screening panel, most commonly the “quad screen,” which also measures hCG, unconjugated estriol, and inhibin A. By combining four markers with the mother’s age, labs can generate a composite risk estimate for both neural tube defects and chromosomal conditions. Each marker contributes different information: hCG tends to be high in Down syndrome pregnancies, estriol tends to be low, and inhibin A adds further discrimination. MSAFP is the marker that contributes most to neural tube defect detection, while its role in Down syndrome screening is as one signal among several.
The quad screen improved detection rates over MSAFP alone, but it still misses a meaningful fraction of affected pregnancies. Depending on the condition, traditional serum screening fails to detect between 5 and 20 percent of Down syndrome cases.10PubMed. Prenatal Screening and Diagnosis: Time for a Paradigm Shift That gap has driven the adoption of newer screening approaches.
What Happens After an Abnormal Result
An abnormal MSAFP result is a screening flag, not a diagnosis. The next steps typically depend on whether the value is high or low and what the clinical picture looks like.
For elevated MSAFP, the usual first move is a detailed ultrasound to check gestational dating, rule out twins, and look directly for structural abnormalities. Modern high-resolution ultrasound has become remarkably sensitive for neural tube defects. In one study comparing ultrasound to amniotic fluid AFP testing, detailed ultrasound detected 100% of neural tube defects and ventral wall defects, while amniotic fluid AFP combined with a secondary marker (acetylcholinesterase) detected only 22 to 77% depending on the condition.11PubMed. Routine measurement of amniotic fluid alpha-fetoprotein and acetylcholinesterase: the need for a reevaluation This has led many centers to rely primarily on ultrasound for the diagnostic workup of elevated MSAFP, reserving amniocentesis for cases where ultrasound findings are ambiguous.
For low MSAFP flagged as part of a multi-marker screen, genetic counseling and possible amniocentesis or chorionic villus sampling are offered to determine fetal chromosomes. Cell-free DNA screening is increasingly offered as an intermediate step before invasive testing.
Unexplained Elevations and Pregnancy Complications
Sometimes MSAFP comes back elevated, but the ultrasound is completely normal and there is no identifiable fetal anomaly. These “unexplained” elevations are not necessarily benign. Research has linked unexplained high MSAFP to a range of later pregnancy complications, including preterm birth, low birth weight, preeclampsia, placental abruption, and fetal death.12American Journal of Obstetrics & Gynecology. Risks of adverse obstetric outcomes in pregnancies with unexplained elevations of maternal serum α-fetoprotein and human chorionic gonadotropin The association with fetal death was particularly strong, with roughly a five-fold increase in risk in both high-risk and low-risk women.
The likely explanation involves the placenta. Elevated AFP may reflect microscopic damage or abnormal development in the placenta that allows more AFP to cross into the maternal circulation than normal. Even when the fetus looks structurally fine on ultrasound, the placenta may not be functioning optimally. This does not mean that every unexplained elevation leads to a bad outcome. Most of these pregnancies proceed normally. But it does mean that providers often increase surveillance, with additional ultrasounds to monitor fetal growth and closer attention to signs of preeclampsia or other placental problems.13Yonsei Medical Journal. Uterine artery Doppler velocimetry in the prediction of adverse obstetric outcomes in unexplained MSAFP elevations
How Ultrasound and Cell-Free DNA Have Changed the Landscape
The role of MSAFP has narrowed substantially over the past two decades. For neural tube defects, mid-trimester ultrasound has largely taken over the screening role that MSAFP once held. The combination of improved ultrasound resolution and widespread routine scanning means that most open neural tube defects are now detected visually rather than through blood chemistry.14PubMed Central. Prenatal screening for neural tube defects: from maternal serum alpha-fetoprotein to ultrasonography
For chromosomal conditions, cell-free DNA (cfDNA) testing, also called noninvasive prenatal testing or NIPT, has emerged as a more sensitive alternative. By analyzing fragments of fetal DNA circulating in the mother’s blood, cfDNA screening detects over 99% of Down syndrome pregnancies, a marked improvement over traditional serum screening.15PubMed. Prenatal Screening and Diagnosis: Time for a Paradigm Shift The introduction of cfDNA screening has already shifted patterns in prenatal care: referrals for genetic counseling because of abnormal maternal serum screening have dropped significantly, while referrals for other indications have risen.16PubMed. Introduction of cell-free DNA screening is associated with changes in prenatal genetic counseling indications
That said, MSAFP has not disappeared from clinical practice. It remains the only second-trimester blood marker specifically designed to flag open neural tube defects. CfDNA testing screens for chromosomal abnormalities, not structural defects. A provider who orders only cfDNA and skips MSAFP may miss the neural tube defect screening component entirely. Some professional guidelines recommend that even when cfDNA is used for aneuploidy screening, MSAFP or a detailed second-trimester ultrasound should still be performed to cover the structural-defect gap.
The Emotional Weight of an Abnormal Screen
Getting a call that your MSAFP is abnormal can be deeply unsettling, and research suggests that the anxiety it generates is real and measurable. Studies comparing women referred for genetic testing because of abnormal MSAFP versus women referred because of advanced maternal age found that the MSAFP group experienced higher anxiety levels, even though the actual genetic risks in both groups were comparable.17PubMed. Determinants of altered anxiety after abnormal maternal serum alpha-fetoprotein screening Part of the explanation may be how risk is perceived: women with advanced maternal age tend to see the risk as belonging to their age group in general, while women with an abnormal MSAFP screen perceive it as a personal, individual risk directed at their specific pregnancy.18PubMed. To Use or Not to Use: The Prenatal Genetic Technology/Worry Conundrum
There is also a documented knowledge gap. Research has found more misunderstanding and misinformation about MSAFP screening compared to amniocentesis. Women often struggle with the concept that a screening test gives a probability, not a definitive answer. Many women in these studies expressed being torn between fear of an invasive diagnostic procedure and worry about spending the rest of the pregnancy not knowing whether the baby was healthy.19PubMed. To Use or Not to Use: The Prenatal Genetic Technology/Worry Conundrum Good genetic counseling before the blood draw, explaining what the test can and cannot tell you and what the possible next steps would be, goes a long way toward reducing that distress.
Equity and Access in Prenatal Screening
The availability and uptake of prenatal screening, including MSAFP, is not uniform across socioeconomic groups. A large national survey in France found significant socioeconomic barriers to informed decision-making about maternal serum screening for Down syndrome. Women with fewer economic resources were less likely to receive adequate information, less likely to be offered screening, and less likely to have access to the full range of follow-up options.20PubMed Central. Socioeconomic Barriers to Informed Decisionmaking Regarding Maternal Serum Screening for Down Syndrome: Results of the French National Perinatal Survey of 1998 The researchers noted that unless these disparities are addressed, the burden of caring for children with congenital conditions detected by screening will disproportionately fall on families with the fewest resources. While that study focused on France, similar patterns exist in other countries where screening is offered but access to counseling and follow-up is uneven.
Folic Acid and Why Prevention Matters Alongside Screening
Screening for neural tube defects is important, but preventing them in the first place has arguably had a bigger public health impact. Folic acid supplementation before and during early pregnancy has been shown to reduce the occurrence of neural tube defects by 50 to 70%.21PubMed Central. Epidemiology, prenatal management, and prevention of neural tube defects Several countries have adopted mandatory folic acid fortification of staple foods like flour and cereal grains, specifically to reach women with unplanned pregnancies and those who might not take supplements on their own.
The relationship between folic acid fortification and MSAFP screening is an interesting one. As neural tube defect rates have fallen in countries with robust fortification programs, the proportion of elevated MSAFP results that turn out to be caused by actual neural tube defects has also dropped. A higher fraction of flagged results now fall into the “unexplained” category, meaning the positive predictive value of an elevated MSAFP for neural tube defects is lower than it was when the test was first introduced. This is one more reason why ultrasound has become the preferred method for directly evaluating whether a structural defect is present after an abnormal blood result.
AFP Expression in Early Embryonic Development
Research has also explored AFP at the very earliest stages of development, before a pregnancy is even established in the uterus. A proof-of-concept study examined fluid from human blastocysts (the stage an embryo reaches around day five after fertilization, before implantation) and found that a small fraction of blastocysts expressed detectable AFP. About 12.5% of samples showed measurable AFP, though at extremely low concentrations measured in picograms per milliliter. Among blastocysts that expressed AFP, 80% were aneuploid (had an abnormal number of chromosomes), though the sample was too small to establish a statistical link between AFP expression and aneuploidy.22International Journal of Molecular Sciences. The Expression of Alpha-Fetoprotein in Human Blastocoel Fluid-Conditioned Media In Vitro: A Proof of Concept Study The finding is preliminary but raises the question of whether AFP could someday play a role in embryo selection during IVF, adding biological markers to the genetic testing already used.