Final answer:
The AFP test is used during pregnancy to detect neural tube defects in the fetus by measuring AFP levels in the mother's blood.
Step-by-step explanation:
The rationale for the antepartum test called AFP (Alpha-fetoprotein) is to detect neural tube defects in the fetus. Neural tube defects are birth defects that occur when the neural tube, which forms the baby's brain and spinal cord, fails to close properly during early pregnancy. By measuring AFP levels in the mother's blood, doctors can identify if there is an increased risk of neural tube defects in the fetus.