The alfa-fetoprotein (AFP or MSAFP) test is a maternal blood test done in the second trimester that checks for a protein normally secreted by the fetal liver. The levels of this protein alert the obstetrician to the possibility of a chromosomal abnormality, such as Down syndrome or the presence of twins. Abnormal AFP levels may also indicate developmental problems in the fetus, such as neural tube defects like spina bifida or defects in the abdominal wall of the fetus.

An abnormal AFP may also simply mean that your due date has been miscalculated. Therefore, an abnormal AFP indicates the need for further testing. AFP testing is often done in concert with testing for other markers, such as hCG, estriol and inhibin, three hormones produced by the placenta.

This is called multiple marker screening (sometimes called the "triple screen"), but abnormal results do not necessarily mean your pregnancy has a problem. This is why these are considered screening tests – typically other tests, such as an amniocentesis or an ultrasound have to be done to diagnose a problem, if one exists.

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