Cautions

Race, weight, smoking, multiple fetus pregnancy, and insulin-dependent diabetes (IDD) may affect marker concentrations. Black mothers tend to have higher alpha-fetoprotein (AFP) levels but lower risk of neural tube defects and are assigned to a separate AFP median set. Multiple of the medians (MoM) are adjusted for maternal weight (to account for dilution effects in heavier mothers). The AFP is adjusted upward in IDD to account for lower values in diabetic pregnancies. Smoking results in higher second trimester maternal serum AFP. MoM are adjusted accordingly to account for serum AFP differences in smokers.

The screen results are dependent on accurate information for gestation, race, IDD, and weight. Inaccurate information can lead to significant alterations in the estimated risk. In particular, erroneous assessment of gestational age can result in false-positive or false-negative screen results. Because of its increased accuracy, the determination of gestational age by ultrasound is recommended, when possible, rather than by last menstrual period.

A screen-negative result does not guarantee the absence of fetal defects. A screen-positive result does not provide a diagnosis but indicates that further diagnostic testing should be considered (an unaffected fetus may have screen-positive result for unknown reasons).

Valid measurements of AFP in maternal serum cannot be made after amniocentesis.

Triplet and higher multiple pregnancies cannot be interpreted.

Each center offering maternal serum screening to patients should establish a standard screening protocol, which provides pre- and post-screening education and appropriate follow-up for screen-positive results.

In rare cases, some individuals can develop antibodies to mouse or other animal antibodies (often referred to as human anti-mouse antibodies [HAMA] or heterophile antibodies), which may cause interference in some immunoassays. Caution should be used in interpretation of results, and the laboratory should be alerted if the result does not correlate with the clinical presentation.