Alpha Fetoprotein (AFP)Tumor Marker and Role in Pregnancy

Alpha fetoprotein (AFP) is a major plasma glycoprotein that plays a crucial role during embryonic and fetal development. It was first identified in the 1950s as a fetal counterpart of adult serum albumin. Synthesized primarily by the fetal liver, yolk sac, and gastrointestinal tract, AFP is one of the most abundant proteins in fetal circulation but declines rapidly after birth, remaining at very low concentrations in healthy adults.

In clinical medicine, AFP has gained prominence as both a biochemical marker of fetal health and a tumor marker in adults. Elevated or decreased levels of AFP can provide valuable diagnostic information in various physiological and pathological conditions. For example, maternal serum AFP (MSAFP) testing is widely used for prenatal screening of neural tube defects and chromosomal abnormalities, while in adults, serum AFP testing serves as a key biomarker in hepatocellular carcinoma (HCC) and germ cell tumors.

This article explores the structure, biological role, and clinical applications of alpha fetoprotein, with a focus on its diagnostic importance in pregnancy and oncology. You will also learn about AFP testing, interpretation of results, and recent advances in biomarker research that enhance the precision of AFP-based diagnostics.

Biological Role and Function of Alpha Fetoprotein

Alpha fetoprotein (AFP) is an important fetal serum protein that serves multiple physiological functions during early development. Structurally related to serum albumin, AFP belongs to the albuminoid gene family, which includes albumin and vitamin D-binding protein. Despite their structural similarities, AFP exhibits unique roles that are essential for the growth and protection of the developing fetus.

1. Sites of Synthesis

During embryogenesis, AFP is synthesized primarily by the yolk sac, and later by the fetal liver and gastrointestinal tract. The production of AFP begins as early as the fourth week of gestation, reaching peak concentrations between the 12th and 16th weeks of pregnancy. After birth, AFP synthesis decreases rapidly, and serum levels fall to adult reference ranges within the first year of life.

2. Physiological Functions

Although not completely understood, several important biological functions of AFP have been identified:

• Carrier Protein: AFP binds and transports a wide range of ligands, including bilirubin, fatty acids, steroids, and drugs, similar to the role of albumin in adults.
• Regulation of Fetal Growth: AFP contributes to maintaining osmotic pressure and nutrient transport in fetal plasma, supporting tissue growth and differentiation.
• Immunomodulation: AFP has immunosuppressive properties that help protect the fetus from maternal immune attack, preventing the maternal immune system from recognizing fetal tissues as foreign.
• Cell Growth and Differentiation: Experimental evidence suggests AFP may act as a growth regulator by interacting with cell surface receptors and influencing gene expression during organogenesis.

3. AFP Levels Across Development

In fetal life, serum AFP concentrations can reach several thousand nanograms per milliliter, making it one of the most abundant plasma proteins in the fetus. After birth, levels decline dramatically — typically dropping below 10 ng/mL in healthy adults. Persistently high AFP levels in adults may indicate liver pathology or malignancy, highlighting its transition from a developmental protein to a clinical biomarker.

Alpha Fetoprotein in Pregnancy

During pregnancy, alpha fetoprotein (AFP) plays an important role as both a fetal serum protein and a maternal screening marker. AFP is produced by the fetal liver and yolk sac, and a small portion of it crosses the placenta into the maternal bloodstream, where it can be measured to assess fetal development and detect abnormalities.

1. Maternal Serum AFP (MSAFP) Test

The maternal serum alpha fetoprotein (MSAFP) test is a key component of prenatal screening programs, usually performed between the 15th and 20th weeks of gestation. This test measures the concentration of AFP in the mother’s blood and compares it with standard reference values adjusted for gestational age.

The test may be performed as part of the:

• Triple test (AFP, hCG, and estriol)
• Quadruple test (AFP, hCG, estriol, and inhibin A)

These screening tests help estimate the risk of certain fetal conditions and guide further diagnostic procedures when abnormal results are found.

2. Interpretation of AFP Levels in Pregnancy

The interpretation of AFP levels is based on comparison to the median for gestational age, expressed as multiples of the median (MoM).

• Elevated AFP levels (≥ 2.5 MoM) may indicate open neural tube defects (spina bifida, anencephaly), abdominal wall defects, multiple gestation, or incorrect gestational dating.
• Low AFP levels can be associated with chromosomal abnormalities, including Down syndrome (trisomy 21) and Edward’s syndrome (trisomy 18).

It is important to note that abnormal AFP results do not confirm a diagnosis but rather indicate a need for further evaluation through ultrasound or amniocentesis.

3. Clinical Significance

Measurement of maternal serum alpha fetoprotein is one of the earliest and most effective methods for noninvasive prenatal screening. When combined with other biochemical markers and fetal imaging, AFP testing contributes to the early detection of congenital disorders and improves maternal-fetal health outcomes.

Alpha Fetoprotein as a Tumor Marker

Beyond its developmental role, alpha fetoprotein (AFP) serves as a valuable tumor marker in clinical oncology. Its reappearance or sustained elevation in adults—where AFP is normally found at very low levels—often indicates pathological reactivation of fetal gene expression, a hallmark of certain malignancies.

1. AFP and Hepatocellular Carcinoma (HCC)

One of the most significant clinical applications of AFP testing is in the diagnosis and monitoring of hepatocellular carcinoma (HCC), the most common primary liver cancer.

• Elevated AFP levels (often >400 ng/mL) are observed in many HCC patients and can support diagnosis when combined with imaging studies such as ultrasound, CT, or MRI.
• AFP is also used to monitor treatment response and detect tumor recurrence after surgery, ablation, or liver transplantation.
  However, mild elevations can also occur in chronic hepatitis and liver cirrhosis, limiting AFP’s specificity when used alone.

2. AFP in Germ Cell Tumors

Alpha fetoprotein is also an established tumor marker for germ cell malignancies, particularly those arising in the testes and ovaries.

• Non-seminomatous testicular tumors (e.g., yolk sac tumors, embryonal carcinomas) frequently produce AFP, whereas pure seminomas do not.
• In women, ovarian yolk sac tumors are similarly associated with marked AFP elevation.
  Measurement of AFP, often in combination with human chorionic gonadotropin (hCG) and lactate dehydrogenase (LDH), assists in diagnosis, prognosis, and therapy monitoring for these cancers.

3. Diagnostic and Prognostic Value

AFP’s clinical value extends beyond detection. Persistent or rising AFP levels during or after treatment may indicate disease progression or recurrence, while decreasing levels suggest therapeutic response.
Modern laboratories also measure AFP isoforms—such as AFP-L3, a glycoform more specific for malignant hepatocytes—to improve diagnostic accuracy in liver cancer.

4. Limitations

Although AFP remains a useful biomarker, its diagnostic performance is not absolute:

• False positives may occur in benign liver conditions.
• False negatives can appear in small or poorly differentiated tumors that do not secrete AFP.
  For this reason, AFP testing is most effective when integrated into a multimodal diagnostic approach combining biochemical markers, imaging, and clinical evaluation.

Conclusion

Alpha fetoprotein (AFP) is a versatile biomarker with significant importance in both developmental biology and clinical diagnostics. Initially vital for fetal growth and protection, AFP becomes a key tumor marker and prenatal screening tool later in medical practice. Its measurement provides valuable information for detecting liver cancer, germ cell tumors, and fetal abnormalities, highlighting its dual relevance in obstetrics and oncology.
While AFP testing has some limitations, ongoing research into AFP isoforms and molecular profiling continues to enhance its diagnostic precision and clinical utility in modern medicine.

References:

1. Yeo YH, Lee YT, Tseng HR, Zhu Y, You S, Agopian VG, Yang JD. Alpha-fetoprotein: Past, present, and future. Hepatol Commun. 2024 Apr 12;8(5):e0422. doi: 10.1097/HC9.0000000000000422.
2. Niaki NM, Hatefnia F, Heidari MM, Tabean M, Mobed A. Alpha-Fetoprotein (AFP) biosensors. Clin Chim Acta. 2025 Jun 1;573:120293. doi: 10.1016/j.cca.2025.120293.
3. Hu X, Chen R, Wei Q, Xu X. The Landscape Of Alpha Fetoprotein In Hepatocellular Carcinoma: Where Are We? Int J Biol Sci. 2022 Jan 1;18(2):536-551. doi: 10.7150/ijbs.64537.
4. Bei R, Mizejewski GJ. Alpha-fetoprotein is an autoantigen in hepatocellular carcinoma and juvenile Batten disease. Front Biosci (Landmark Ed). 2020 Jan 1;25(5):912-929. doi: 10.2741/4840.
5. Pak VN. α-fetoprotein-binding toxins and teratogens against cancer. Ther Deliv. 2019 Jan;10(1):1-3. doi: 10.4155/tde-2018-0068.
6. Zheng Y, Zhu M, Li M. Effects of alpha-fetoprotein on the occurrence and progression of hepatocellular carcinoma. J Cancer Res Clin Oncol. 2020 Oct;146(10):2439-2446. doi: 10.1007/s00432-020-03331-6.
7. Matonóg A, Drosdzol-Cop A. Alpha-fetoprotein level in fetuses, infants, and children with ovarian masses: a literature review. Front Endocrinol (Lausanne). 2024 Feb 6;15:1307619. doi: 10.3389/fendo.2024.1307619.