A protein that is normally synthesized by the cells of the yolk sac during pregnancy, and from the 5th week of pregnancy by the fetal liver. It is excreted into the amniotic fluid in the urine, then absorbed through the membranes into the pregnant woman's bloodstream. It begins to be detected in the mother's blood from the 10th week and increases steadily until the 25th week of pregnancy, and then gradually decreases. During the first year, AFP decreases to adult levels.