Blood was collected into EDTA tubes, kept on ice, and centrifuged (1500 x g, 20 min) at 4 ̊C within 60 min. Separated plasma was stored at ^–80°C until further processing. The 200μL plasma samples were analyzed by the Genome Technology Access Center (St. Louis, MO) using a highly multiplexed, aptamer-based platform capturing 1310 proteins (SomaLogic, Inc., Boulder, CO). The assay quantifies proteins over a wide dynamic range (> 8 log) using chemically modified aptamers with slow off-rate kinetics (SOMAmer reagents). Each SOMAmer reagent is a unique, high-affinity, single-strand DNA endowed with functional groups mimicking amino acid side chains. In brief, samples were incubated on 96-well plates with a mixture of SOMAmer reagents. Two sequential bead-based immobilization and washing steps were used to eliminate nonspecifically-bound proteins, unbound proteins, and unbound SOMAmer reagents from protein target-bound reagents. After eluting SOMAmer reagents from the target proteins, the fluorescently-labeled reagents were quantified on an Agilent hybridization array (Agilent Technologies, Santa Clara, CA). Data were normalized in 4 specific steps and according to assay data quality control procedures defined in the good laboratory practice quality system of SomaLogic, Inc. Normalization steps control for signal intensity biases introduced by differential hybridization efficiencies and the overall brightness of plates, collection protocol artifacts, and batch effects between different plates.

The data files contain the normalized plasma protein data, expressed as relative fluorescence units (RFU). Please refer to the ReadMe file ('Onset of labor proteomics') for additional information.