AFP and preeclampsia: While the measurement of circulating proteins may offer more consistency than RNA species, abnormal levels of placental protein 13 (PP13), pregnancy-associated plasma protein A (PAPP-A), Alpha-fetoprotein (AFP), and growth differentiation factor 15 (GDF-15), rather than placental growth factor (PIGF) alone or the soluble fms-like tyrosine kinase 1/placental growth factor (sFlt-1/PIGF) ratio, are associated with adverse pregnancy outcomes, including preeclampsia [40,41].