During pregnancy, a developing fetus produces a protein called Alpha-fetoprotein (AFP). This protein, made by the fetal liver, circulates in the baby’s bloodstream and enters the mother’s blood supply. A maternal serum AFP test is a blood test offered between the 15th and 20th weeks of gestation to measure this protein’s level. This is a screening test, not a diagnostic one. It identifies pregnancies with an increased risk for certain birth defects that may warrant further evaluation.
How the Normal Range is Calculated
The “normal range” for AFP levels is not a single, fixed value. Instead, it is highly individualized and reported as a Multiple of the Median (MoM). The MoM compares an individual’s AFP level to the median AFP level of other pregnancies at the same gestational age. This approach is necessary because the concentration of AFP in a mother’s blood changes weekly throughout the second trimester.
Several personal factors are integrated into the MoM calculation to establish a precise, adjusted normal range. The most significant factor is the gestational age of the fetus; an inaccurate due date is a common reason for an unusual result. Maternal weight is also included, as AFP can be diluted in the bloodstream of heavier individuals. A person’s race and whether they have pre-existing diabetes that requires insulin are also considered.
A result is provided as a value, such as 1.0 MoM, which indicates the AFP level is exactly the median for that stage of pregnancy. A result of 2.0 MoM means the level is twice the median, while a 0.5 MoM result means it is half the median. Laboratories establish a specific cutoff MoM value, often around 2.0 or 2.5, to distinguish between a screen-negative and a screen-positive result. A result above this cutoff is considered high, while one below a lower threshold is considered low.
Reasons for High AFP Levels
An elevated AFP result does not automatically signify a problem with the pregnancy. One common explanation for higher-than-expected AFP levels is the presence of more than one fetus. In a multiple gestation, such as with twins or triplets, each baby produces AFP, contributing to a higher total concentration in the maternal bloodstream.
In some cases, a high AFP level can indicate an increased risk for specific types of birth defects. These primarily include open neural tube defects (ONTDs), such as spina bifida, where the fetal spinal column does not close completely during development. It may also suggest the presence of an abdominal wall defect, like gastroschisis or omphalocele, where a portion of the fetal intestines or other organs protrude through an opening in the abdomen.
Reasons for Low AFP Levels
A low AFP MoM value points to a different set of potential concerns. When the gestational age is confirmed to be accurate, a low AFP level is primarily associated with an increased risk for certain chromosomal abnormalities. The most notable of these is Trisomy 21, also known as Down syndrome. Another condition linked to low AFP results is Trisomy 18, or Edwards syndrome, a more severe and less common chromosomal disorder.
Low AFP is just one marker used in prenatal screening. It is often part of a larger panel of blood tests, like the triple or quad screen, which measures other substances like hCG, estriol, and inhibin-A. The combination of these results with maternal age provides a more comprehensive risk assessment for chromosomal conditions than the AFP level alone.
Diagnostic Steps After an Abnormal Result
Receiving an abnormal AFP result initiates a clear pathway of follow-up steps to gather more conclusive information. The first step is often a consultation with a genetic counselor. These specialists can explain the screening results in detail, discuss the specific risks indicated, and outline the available options for further testing.
The next action is a detailed, high-resolution ultrasound, sometimes called a Level II ultrasound. This imaging procedure serves two main purposes: it verifies the gestational age of the fetus and allows for a thorough examination of the fetal anatomy. The sonographer will check the baby’s spine, brain, and abdominal wall for any visible abnormalities and confirm if there is more than one baby. This ultrasound can provide reassurance and clarify the reason for the initial result.
If uncertainty persists after the ultrasound, a diagnostic test may be offered. The most common of these is amniocentesis. This procedure involves collecting a small sample of the amniotic fluid that surrounds the fetus. By analyzing the fetal cells contained in this fluid, amniocentesis can definitively diagnose or rule out chromosomal abnormalities and can also measure AFP levels directly from the source, providing more accurate information regarding the risk of open neural tube defects.