The MSAFP test is a blood draw that measures the level of alpha-fetoprotein, a protein produced by a developing fetus, circulating in a pregnant person’s bloodstream. Performed during the second trimester, it screens primarily for neural tube defects like spina bifida and anencephaly, though abnormal results can also flag chromosomal conditions and other structural problems. The test has been a standard part of prenatal care for roughly four decades, and while newer screening technologies have emerged alongside it, MSAFP remains one of the most widely used and cost-effective tools in prenatal medicine.
What Alpha-Fetoprotein Is and Why It Ends Up in Your Blood
Alpha-fetoprotein (AFP) is a protein made mainly by the fetal liver. During pregnancy, AFP passes from the fetus into the amniotic fluid and then crosses the placenta into the mother’s bloodstream. The amount of AFP in maternal blood rises steadily as pregnancy progresses, peaking around the end of the second trimester before declining. Because AFP levels change quickly week by week, the timing of the blood draw matters a great deal for accurate interpretation.
AFP plays several roles during fetal development, including involvement in fetal blood cell formation and organ development. It has been described as something of a molecular “troubleshooter” during pregnancy, linked to processes ranging from thyroid function to folate metabolism. For screening purposes, though, the key insight is simple: when certain fetal abnormalities are present, the amount of AFP that leaks into the mother’s blood shifts in predictable directions. Open defects in the fetal body tend to push levels up, while certain chromosomal conditions tend to push them down.
When the Test Is Done
The standard window for MSAFP screening is 15 to 18 weeks of gestation, early in the second trimester. This timing has been considered the gold standard for over four decades.1PubMed Central. Prenatal screening for neural tube defects: from maternal serum alpha-fetoprotein to ultrasonography The test itself is straightforward: a single tube of blood is drawn, and the sample is sent to a lab that measures how much AFP is present in the serum. Results typically come back within a week or two.
In many practices, the MSAFP is offered as part of a broader panel rather than on its own. When combined with measurements of other markers like hCG and estriol, it becomes what’s known as the “triple screen” or, with the addition of inhibin-A, the “quad screen.” These additional markers improve the ability to detect chromosomal conditions like Down syndrome. But the AFP component remains the workhorse for detecting neural tube defects specifically.
How Results Are Reported
Raw AFP numbers are not particularly useful by themselves because normal levels vary depending on how far along the pregnancy is, the mother’s weight, whether she has diabetes, and her racial background. To make results comparable, labs convert the raw measurement into “multiples of the median,” or MoM. A result of 1.0 MoM means your AFP level sits right at the median for your gestational age. Values above or below that midpoint are where clinical decisions begin.
The weight correction is especially meaningful. A heavier person has a larger blood volume, which dilutes the AFP concentration. Without adjusting for this, a heavier person might get a falsely reassuring low reading. Diabetes adds another layer. In people with type 1 diabetes, AFP values tend to run lower than average. One study found that without any corrections, AFP in people with insulin-dependent diabetes averaged about 0.91 MoM; applying weight correction alone brought it to 0.96 MoM, and applying both weight and the older diabetes correction factor actually overcorrected it to 1.20 MoM.2American Journal of Obstetrics and Gynecology. Correction for insulin-dependent diabetes in maternal serum α-fetoprotein testing has outlived its usefulness Both type 1 and type 2 diabetes require adjustments to bring MSAFP values in line with non-diabetic levels.3PubMed. Maternal serum alpha-fetoprotein values in type 1 and type 2 diabetic patients Getting these corrections wrong can lead to missed diagnoses or unnecessary invasive testing.
What High Levels Can Mean
An MSAFP level above about 2.0 to 2.5 MoM triggers further evaluation. The classic reason for a high reading is a neural tube defect, a category that includes spina bifida (an incomplete closure of the spinal column), anencephaly (absence of major portions of the brain and skull), and encephalocele (brain tissue protruding through an opening in the skull). When these defects are “open,” meaning the tissue is exposed rather than covered by skin, AFP leaks directly from the fetus into the amniotic fluid and then into the maternal bloodstream at elevated rates.
Abdominal wall defects are another well-established cause of high MSAFP. Gastroschisis, where the intestines protrude through an opening in the abdominal wall, produces especially dramatic elevations. One study found a median AFP of 7.0 MoM in gastroschisis cases, while omphalocele, a related but distinct defect where abdominal contents push into the base of the umbilical cord, produced a median of 4.1 MoM.4PubMed. Second-trimester maternal serum alpha-fetoprotein levels in pregnancies associated with gastroschisis and omphalocele Another study confirmed this pattern, reporting MSAFP of about 4.4 MoM for gastroschisis and 2.3 MoM for omphalocele.5PubMed Central. Maternal serum alpha fetoprotein and free β-hCG of second trimester for screening of fetal gastroschisis and omphalocele Both were far higher than the roughly 1.0 MoM seen in unaffected pregnancies.
Before assuming the worst, though, there are mundane explanations for a high reading. The most common is an inaccurate estimate of gestational age. If you’re further along than thought, your AFP will naturally be higher and could look abnormal for the earlier date. Twin pregnancies also produce higher AFP because two fetuses contribute to the total. These possibilities are typically sorted out quickly with an ultrasound.
What Low Levels Can Mean
Low MSAFP drew attention in the mid-1980s when researchers noticed a link between reduced AFP levels and fetal Down syndrome (trisomy 21). In most pregnancies involving a Down syndrome fetus, maternal serum AFP drops to roughly 70 percent of the level seen in unaffected pregnancies.6PubMed. Reduced fetal hepatic alpha-fetoprotein levels in Down’s syndrome The reduction appears to originate in the fetal liver itself, which produces less AFP when an extra copy of chromosome 21 is present.
This discovery turned what had been a one-directional screening tool into a two-directional one. Early prospective screening programs using low MSAFP detected fetuses with trisomy 21 and trisomy 18 at meaningful rates among women who went on to have amniocentesis.7Pediatric Research. PROSPECTIVE MSAFP SCREENING FOR DOWN SYNDROME: THE BAYLOR EXPERIENCE Low AFP alone is not particularly sensitive for Down syndrome, which is why it was eventually bundled with other serum markers into multi-analyte screens. But it remains a core component of those panels.
What Happens After an Abnormal Result
An abnormal MSAFP is a screening result, not a diagnosis. Most people who get a high or low reading will ultimately have a healthy pregnancy. The first step after an elevated result is almost always a detailed ultrasound to check gestational dating, look for twins, and examine fetal anatomy for visible structural abnormalities. Modern high-resolution ultrasound catches many of the same defects that MSAFP flags, and the two tests complement each other. One study found that MSAFP screening missed a quarter of neural tube defects in screened cases, including nearly 40 percent of spina bifida and two-thirds of encephalocele cases, which ultrasound was able to identify.8Obstetrics & Gynecology. Routine Ultrasonography Compared With Maternal Serum Alpha-fetoprotein for Neural Tube Defect Screening At the same time, some defects that MSAFP catches are subtle enough to be missed on a scan, so the two approaches work best as a pair rather than a replacement for one another.
If ultrasound doesn’t explain the abnormal AFP, amniocentesis may be offered. The amniotic fluid itself can be tested for AFP concentration and for acetylcholinesterase, an enzyme that leaks through open neural defects. As ultrasound technology has improved, some researchers have questioned whether routine amniotic fluid AFP testing at the time of amniocentesis still adds enough value to justify it, particularly given how accurately ultrasound now detects neural tube and ventral wall defects.9PubMed. Routine measurement of amniotic fluid alpha-fetoprotein and acetylcholinesterase: the need for a reevaluation That said, amniocentesis also allows chromosomal analysis, which is crucial when low AFP has raised concern about Down syndrome or trisomy 18.
When No Structural Cause Is Found
Here’s where things get less clear-cut. In a meaningful percentage of cases, MSAFP comes back elevated, ultrasound looks normal, and no fetal abnormality is ever identified. These are labeled “unexplained” elevations, and they’re not entirely benign. Research consistently shows that unexplained high MSAFP is associated with a higher risk of pregnancy complications later on.
A large study found that unexplained elevated MSAFP was independently linked to a range of poor outcomes in both high-risk and low-risk women.10PubMed. Unexplained elevated maternal serum alpha-fetoprotein and/or human chorionic gonadotropin and the risk of adverse outcomes A separate investigation put numbers on the specific risks: compared to women with normal AFP levels, those with high unexplained MSAFP had roughly double the rate of preterm birth, pregnancy-induced hypertension, and fetal growth restriction. The rate of fetal death was more than seven times higher, and low birth weight was about two and a half times more common.11PubMed. Associations between pregnancy outcomes and unexplained high and low maternal serum alpha-fetoprotein levels
The reasons behind these associations are not fully understood. One theory involves placental dysfunction: if the placenta isn’t functioning properly, AFP and other substances may cross into the maternal bloodstream at unusual rates. This would explain why the same pregnancies later develop complications like preeclampsia and growth restriction, both of which involve impaired placental blood flow. Whatever the mechanism, an unexplained high MSAFP typically prompts closer monitoring for the remainder of the pregnancy, including more frequent ultrasounds and possibly additional fetal surveillance in the third trimester.
The Emotional Weight of an Abnormal Screen
Screening tests exist in a difficult space. They cast a wide net, which means many people who receive abnormal results will go on to have perfectly normal pregnancies. But the days or weeks spent waiting for follow-up testing can be genuinely distressing. Research looking at the psychological effects of false-positive prenatal screening results found that receiving an abnormal result was associated with high anxiety, increased worry about the baby’s health, and a more negative attitude toward the pregnancy.12PubMed. The psychological effects of false-positive results in prenatal screening for fetal abnormality: a prospective study
Low AFP results carry their own particular burden. One study compared anxiety levels in women undergoing amniocentesis because of low MSAFP with those doing so because of advanced maternal age. The women flagged by low AFP were more anxious, possibly because a screening result that suggests Down syndrome can feel more alarming and unexpected than a risk factor someone has been aware of from the start.13PubMed. Anxiety in women with low maternal serum alpha-fetoprotein screening results Good genetic counseling before and after screening can help, but the emotional reality of an abnormal result is worth taking seriously. If you’re being offered MSAFP or a multi-marker screen, understanding that an abnormal result is a probability statement rather than a diagnosis can take some of the sting out of the waiting period should the results come back flagged.
How Folic Acid Changed the Landscape
In the late 1990s, the United States mandated the fortification of enriched grain products with folic acid to reduce the incidence of neural tube defects. The impact on MSAFP screening has been striking. One analysis found a 32 percent decrease in the proportion of pregnant people with high MSAFP values after fortification took effect, reflecting a real decline in neural tube defect rates.14PubMed. Impact of folic acid fortification in the United States: markedly diminished high maternal serum alpha-fetoprotein values
This shift has ripple effects for how MSAFP screening performs. Fewer genuine neural tube defect cases in the population means a greater proportion of high MSAFP results will be false positives, which changes the positive predictive value of the test. It doesn’t make the test useless by any means. Neural tube defects have not been eliminated, and MSAFP still catches abdominal wall defects and flags pregnancies at risk for later complications. But folic acid fortification represents one of those rare public health wins that actually reduces the burden a screening test was designed to catch.
Is the Test Worth the Cost
From a health-economics standpoint, MSAFP screening has consistently been shown to pay for itself. An early cost-benefit analysis calculated that screening 100,000 women cost just over $2 million but produced economic benefits exceeding $4 million.15PubMed Central. Maternal serum alpha-fetoprotein screening: a cost-benefit analysis The favorable ratio holds even after accounting for follow-up ultrasounds and amniocentesis in screen-positive cases.
A later analysis compared different strategies for evaluating pregnancies with elevated MSAFP. Using targeted ultrasound for results at or above 2.0 MoM detected the large majority of structurally abnormal fetuses without any procedure-related fetal loss, at a relatively low cost per anomaly detected. Adding amniocentesis with chromosome analysis for results at or above 2.5 MoM picked up additional abnormalities, including chromosomal conditions, but at a substantially higher incremental cost and with some risk of procedure-related loss.16PubMed. Cost-effectiveness of strategies used in the evaluation of pregnancies complicated by elevated maternal serum alpha-fetoprotein levels California’s triple marker program, one of the largest statewide prenatal screening programs, remained cost-beneficial at a ratio of roughly 2.7 to 1.17Genetics in Medicine. Cost and effectiveness of the California triple marker prenatal screening program
MSAFP Alongside Newer Screening Options
Cell-free DNA screening (sometimes called NIPT, for noninvasive prenatal testing) has become widely available and is highly accurate for detecting common chromosomal conditions like Down syndrome, trisomy 18, and trisomy 13. This has led some to ask whether MSAFP is still necessary. The answer depends on what you’re screening for. Cell-free DNA is excellent at chromosomal screening but does not detect neural tube defects or abdominal wall defects. It also doesn’t produce the unexplained elevations that, as discussed above, serve as an early warning for placental problems and later pregnancy complications.
Some practices now offer NIPT early in pregnancy for chromosomal screening and then add a standalone AFP test in the second trimester specifically for neural tube defect screening. Others continue to use the traditional quad screen. There is no single correct approach, and practice patterns vary by region, insurance coverage, and patient preference. What’s clear is that AFP measurement still fills a niche that newer tests don’t cover. Even as prenatal ultrasound has become remarkably detailed, AFP screening catches some cases that imaging misses and provides information about placental health that no other routine test offers.
For anyone facing a decision about whether to pursue MSAFP screening, the basic trade-off is straightforward: a single blood draw with no physical risk to the pregnancy, balanced against the possibility of an abnormal result that requires follow-up testing and may cause anxiety even if everything turns out fine. Most professional organizations continue to recommend that all pregnant people be offered some form of neural tube defect screening, and for now, measuring AFP in maternal serum remains one of the simplest ways to do that.