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Alpha-fetal protein (AFP) is an alpha-1 globulin secreted by the fetal gastrointestinal tract, yolk sac, and liver, but excreted by the kidney into urine, amniotic fluid, and into the maternal bloodstream through the placenta. In normal pregnancy, fetal serum levels of fetal protein peak between 12 and 14 weeks and maternal serum levels peak between 28 and 32 weeks.