Maternal Serum Alpha-Fetoprotein Test
The maternal serum alpha-fetoprotein (AFP) test measures the concentration of this protein in the mother's blood. AFP is a protein produced in the fetus during pregnancy. This test helps assess the risk of the baby having certain congenital anomalies or genetic conditions.
General Description
Alpha-fetoprotein (AFP) is a glycoprotein produced during pregnancy by the fetal liver and the yolk sac, one of the first structures formed during embryonic development. AFP is excreted into the amniotic fluid and reaches the mother's bloodstream through the placenta. The synthesis of alpha-fetoprotein gradually declines as pregnancy progresses and disappears between 6 and 12 months after birth.
The maternal serum AFP test, therefore, measures the concentration of AFP in the pregnant woman's blood. The highest concentration of this protein in fetal serum occurs between weeks 12 and 14 of gestation. In maternal serum, its concentration increases from week 14, reaches its peak between weeks 28 and 32, and then gradually declines until delivery.
When is it indicated?
The AFP test is a non-invasive prenatal procedure used to determine the baby’s risk of having certain genetic disorders or congenital anomalies, as well as to detect irregular conditions in pregnancy. A high level of maternal serum AFP may indicate a risk of:
- Underestimated gestational age
- Very low maternal weight
- Multiple pregnancy
- Neural tube defects, such as spina bifida, anencephaly, or encephalocele
- Fetal death
- Chromosomal abnormalities, such as Turner syndrome
- Gastroschisis or omphalocele (abdominal wall defects)
- Other congenital disorders, such as congenital nephrosis, esophageal or duodenal atresia, fetal liver necrosis, or hydrocephalus
On the other hand, a decreased AFP concentration may indicate:
- Overestimated gestational age
- Very high maternal weight
- Maternal gestational diabetes
- Molar pregnancy
- Chromosomal abnormalities, such as Down syndrome, Edwards syndrome, or Patau syndrome
- Fetal death
The test is recommended for all pregnant women, especially those over 35, those with diabetes, those with a family history of congenital defects, or those who have used substances harmful to the fetus during pregnancy.
How is it performed?
The blood sample for analysis is obtained through a venipuncture, usually from a vein in the forearm. The test is conducted between weeks 14 and 22 of gestation, ideally between weeks 16 and 18. It is typically part of a series of screening tests known as the maternal triple or quadruple screen, which also measures levels of human chorionic gonadotropin (hCG), unconjugated estriol (uE3), and inhibin A.
Risks
Drawing a blood sample does not pose any serious complications, although a small bruise may appear at the puncture site. However, there is a possibility of obtaining a false positive result, meaning that an increased or decreased AFP level does not actually correspond to any disorder. Similarly, normal AFP values do not guarantee a normal pregnancy or a healthy baby.
What to expect from an AFP test
The blood sample collection for this test is a quick and simple outpatient procedure. The puncture is usually performed on a vein in the forearm. Before the puncture, the area is disinfected, and an elastic band is placed to fill the vein and facilitate needle insertion. Once the sample is collected, pressure is applied to the puncture site to stop the bleeding, followed by the placement of a sterile bandage.
It is important to emphasize that the AFP test is not a diagnostic test but a screening test. This means it does not confirm or rule out the presence of anomalies or disorders in the baby; it only indicates the risk of developing them. If abnormal AFP values are detected in the results, more precise prenatal diagnostic tests, such as high-definition ultrasound or amniocentesis, are necessary.
Specialties that request the AFP test
The AFP test is performed by geneticists and is commonly requested in gynecology and obstetrics consultations.
How to prepare
No specific preparation is required for a maternal serum AFP test. However, to properly interpret the results, it is essential to know the gestational age, the mother’s age, weight, and race, as well as whether the mother has insulin-dependent diabetes.
