Updated project metadata. Defective deep placentation, an abnormal transformation of the spiral arteries in the junctional zone of the myometrium, is known to cause significant obstetrical complications such as preeclampsia (PE), fetal growth restriction, and placental infarcts with fetal death. Serological biomarkers to predict and diagnose PE would help antenatal care and reduce the obstetrical complications. To discover biomarkers for prediction of PE, we first performed global proteome profiling of three pairs of maternal plasma samples obtained from the early second trimester pregnant women who subsequently developed PE and controls to identify proteins that were abundant in the patients. We further evaluated the expression changes of PE representing proteins in stored plasma samples of a cohort of subsequently developed PE and their matched controls by MRM assay. We identified both Complement C1s subcomponent (p-value = 0.041) and Protein AMBP (p-value = 0.043) were up-regulated in the cohort of PE plasma samples before manifestation of clinical disease. We propose that these proteins may be involved in the remodeling process of the spiral arteries even before the manifestation of PE. These proteins could be served as potential plasma biomarkers to predict pregnant women at increased risk of developing PE.