Alpha-fetoprotein (AFP) is a protein produced in the liver of a developing fetus. During a baby's development, some AFP passes through the placenta and into the mother's blood. An AFP test measures the level of AFP in pregnant women during the second trimester of pregnancy. Too much or too little AFP in a mother's blood may be sign of a birth defect or other condition. These include:

A neural tube defect, a serious condition that causes abnormal development of a developing baby's brain and/or spine

Down syndrome, a genetic disorder that causes intellectual disabilities and developmental delays

Twins or multiple births, because more than one baby is producing AFP

Miscalculation of due date, because AFP levels change during pregnancy

Pregnancy:

15 weeks' gestation:  16.3 mcg/mL (16.3 mg/L)

16 weeks' gestation:  14.5 mcg/mL (14.5 mg/L)

17 weeks' gestation:  13.4 mcg/mL (13.4 mg/L)

18 weeks' gestation:  12 mcg/mL (12 mg/L)

19 weeks' gestation:  10.7 mcg/mL (10.7 mg/L)

20 weeks' gestation:  8.1 mcg/mL (8.1 mg/L)

Twin pregnancy:  Values are 2 times higher

Blacks:  Values are 15% higher

Type 2 diabetes mellitus:  Values are 20% lower