AFP is protein made by the fetus' liver, in the foetal gastrointestinal (GI) tract and the yolk sac. During pregnancy, AFP crosses into the mother's blood. The level of AFP in the mother's blood can be measured to screen for disorders such as neural tube defects and Down syndrome. The mother's AFP levels tend to be high with neural tube defects such as anencephaly and Spina bifida, and low with Down syndrome.