AFP is the most abundant plasma protein found in the human fetus. Maternal plasma levels peak near the end of the first trimester, and begin decreasing prenatally at that time, then decrease rapidly after birth. Normal adult levels are usually achieved by the age of 8 to 12 months. The function of AFP in adult humans is unknown; however, in rodents it binds estradiol to prevent the transport of this hormone across the placenta to the fetus. The main function of this is to prevent the virilization of female fetuses. As human AFP does not bind estrogen, its function in humans is less clear. The rodent AFP system can be overridden with massive injections of estrogen, which overwhelm the AFP system and will masculinize the fetus. The masculinizing effect of estrogens may seem counter-intuitive since estrogens are critical for the proper development of female secondary characteristics during puberty. However, this is not the case prenatally. Gonadal hormones from the testes, such as testosterone and antimullerian hormone are required to cause development of a phenotypic male. Without these hormones the fetus will develop into a phenotypic female even if genetically XY. The conversion of testosterone into estradiol by aromatase in many tissues may be an important step in masculinization of that tissue. Masculinization of the brain is thought to occur both by conversion of testosterone into estradiol by aromatase, but also by de novo synthesis of estrogens within the brain. Thus, AFP may protect the fetus from maternal estradiol that would otherwise have a masculinizing effect on the fetus, but its exact role is still controversial.
Serum levels
Maternal
In pregnant women, fetal AFP levels can be monitored in urine. Since AFP is quickly cleared from the mother's serum via her kidneys, maternal urine AFP correlates with fetal serum levels, although the maternal urine level is much lower than the fetal serum level. AFP levels rise until about week 32. Maternal serum alpha-fetoprotein screening is performed at 16 to 18 weeks of gestation. If MSAFP levels indicate an anomaly, amniocentesis will follow.
Infants
The normal range of AFP for adults and children is variously reported as under 50, under 10, and under 5 ng/mL. At birth, normal infants have AFP levels 4 or more orders of magnitude above this normal range, that decreases to a normal range over the first year of life. During this time, the normal range of AFP levels spans approximately 2 orders of magnitude. Correct evaluation of abnormal AFP levels in infants must take into account these normal patterns. Very high AFP levels may be subject to hooking, which results in the level being reported significantly lower than the actual concentration. This is important for analysis of a series of AFP tumor marker tests, e.g. in the context of post-treatment early surveillance of cancer survivors, where the rate of decrease of AFP has diagnostic value.
Clinical significance
Measurement of AFP is generally used in two clinical contexts. First, it is measured in pregnant women through the analysis of maternal blood or amniotic fluid as a screening test for certain developmental abnormalities such as aneuploidy. Second, serum AFP level is elevated in people with certain tumors, and so it is used as a biomarker to follow these diseases. Some of these diseases are listed below:
Developmental birth defects associated with elevated AFP
*Omphalocele
*Gastroschisis
* Neural tube defects: ↑ α-fetoprotein in amniotic fluid and maternal serum
