Maternal serum levels of calcyclin and heat shock protein 90 were compared throughout pregnancy from the first trimester till term among women with preeclampsia (PE) and age-matched normotensive pregnant controls (C). Serum samples from two different studies, a nested case-control study embedded in the Rotterdam periconception cohort and the Lepra Study both conducted at the Erasmus MC in Rotterdam. They were collected in the first, second and third trimester of pregnancy in 43 patients with preeclampsia, consisting of 20 early-onset and 23 late-onset preeclampsia, and 46 normotensive pregnant controls. A serum based 2D LC-MS assay on Parallel Reaction Monitoring mode using a high resolution tribrid mass spectrometer was used to quantify both calcyclin and heat shock protein 90.