AFP stands for alpha-fetoprotein, a protein that serves as an important medical marker in two very different settings: pregnancy screening and cancer detection. In a developing fetus, the liver and yolk sac produce large amounts of AFP to support growth. After birth, levels drop rapidly, and healthy adults typically have only about 3 ng/mL circulating in their blood. When AFP shows up at unusual levels, either in a pregnant person’s blood or in a non-pregnant adult, it signals that something worth investigating may be going on.
What AFP Does in the Body
During fetal development, AFP is one of the most abundant proteins in a baby’s bloodstream. It helps regulate cell growth and differentiation, and it plays a role in maintaining placental function. Think of it as a workhorse protein that the fetus needs during its earliest and most rapid growth phases.
Once a baby is born, AFP levels plummet. Within a few months, they fall below 10 ng/mL and eventually settle around 2 to 4 ng/mL in a healthy adult. The protein has a half-life of about five days, meaning the body clears it quickly. Because mature tissues no longer need it, a significant rise in AFP in an adult almost always points to something abnormal, whether that’s liver disease, certain cancers, or pregnancy.
AFP in Prenatal Screening
The most common reason people encounter an AFP test is during pregnancy. Maternal serum AFP (MSAFP) testing measures how much of the baby’s AFP has crossed the placenta into the mother’s blood. It’s typically drawn between 15 and 20 weeks of gestation, with 16 to 18 weeks considered the ideal window.
A high MSAFP result can suggest a neural tube defect, a group of conditions where the baby’s brain or spinal cord doesn’t close properly during development. At the standard cutoff of 2.5 times the median expected value, the test catches roughly 95% of cases of anencephaly (where the brain doesn’t fully form) and 65 to 80% of open spinal defects like spina bifida. That said, when MSAFP comes back elevated, the actual chance of a neural tube defect is only about 2 to 4%. More often, the result is explained by something less serious: the pregnancy is further along than estimated, or the mother is carrying twins.
Elevated MSAFP that isn’t explained by a neural tube defect can still be clinically meaningful. It’s associated with a higher risk of preterm birth, preeclampsia, fetal growth restriction, and placental problems.
A low MSAFP result points in a different direction. It raises the possibility of chromosomal conditions like Down syndrome or Trisomy 18. This is why AFP is rarely interpreted alone during pregnancy. It’s one component of larger screening panels.
AFP as Part of Multi-Marker Screening
Most prenatal screening combines AFP with other blood markers to improve accuracy. The “quad screen” measures four substances: AFP, human chorionic gonadotropin (hCG), unconjugated estriol, and inhibin-A. Each marker shifts in a characteristic pattern depending on the condition. For Down syndrome, AFP and estriol tend to be low while hCG and inhibin-A tend to be high. For neural tube defects, AFP is elevated. By reading the pattern across all four markers, providers get a much more reliable risk estimate than any single test could provide.
An abnormal result on any of these screens is not a diagnosis. It’s a flag for further evaluation, which typically involves a detailed ultrasound, genetic counseling, and sometimes amniocentesis to analyze fetal chromosomes and AFP levels in the amniotic fluid directly.
AFP as a Cancer Marker
In non-pregnant adults, the most important use of AFP testing is in liver cancer surveillance. Because AFP is originally a fetal liver protein, liver cells that become cancerous sometimes start producing it again. This makes AFP one of the go-to blood markers for hepatocellular carcinoma (HCC), the most common type of primary liver cancer.
The standard screening threshold is 20 ng/mL. At that cutoff, the test picks up liver cancer about 41 to 65% of the time and correctly rules it out in 80 to 94% of people who don’t have it. Raising the threshold to 200 ng/mL makes a positive result almost certain to mean cancer (specificity of 99 to 100%), but the test misses the majority of cases at that level, detecting only 20 to 45%.
This tradeoff is the central limitation of AFP as a cancer screening tool. It misses a significant number of liver cancers, especially early-stage ones, and it can be elevated by non-cancerous conditions. Chronic hepatitis and cirrhosis both cause AFP to rise, sometimes substantially, without any cancer being present. For people living with these conditions, doctors often monitor AFP over time and watch for sudden jumps or very high levels rather than relying on a single reading.
AFP and Testicular or Ovarian Tumors
AFP also plays a role in diagnosing and monitoring certain germ cell tumors, which most commonly affect the testicles in younger men but can also occur in the ovaries. About two-thirds of germ cell tumors that contain yolk sac elements (a specific tissue type) produce AFP. Overall, roughly 90% of nonseminomatous germ cell tumors show elevated AFP, elevated hCG, or both before treatment begins.
One clinically useful detail: pure seminoma, another type of testicular cancer, does not raise AFP levels. If a tumor is thought to be seminoma but AFP is elevated, it usually means there’s a nonseminomatous component that changes how the cancer is staged and treated. After treatment, AFP levels are tracked closely. Because the protein clears from the blood with a known half-life of about five days, levels that don’t drop on schedule can indicate that cancer cells remain.
Non-Cancerous Causes of Elevated AFP
Not every elevated AFP result means cancer. Several liver conditions that aren’t malignant can push levels above the normal range. Chronic hepatitis B and C are common culprits, as are cirrhosis from any cause, including alcohol use and fatty liver disease. Active liver inflammation or injury, even from a temporary cause, can temporarily raise AFP. This is one reason the test is never used in isolation to diagnose cancer. Imaging studies, additional blood work, and sometimes biopsy are needed to determine what’s actually driving the elevation.
For people with chronic liver disease who are already at higher risk of developing liver cancer, periodic AFP testing alongside ultrasound remains a standard surveillance approach. The combination of both tests performs better than either one alone, helping catch cancers earlier when they’re more treatable.