The concentrations of resistin in serum and cell culture media were measured using human Resistin DuoSet ELISA kit (R&D Systems, Minneapolis, MN) according to manufacturer’s protocol. Briefly, ELISA plates were coated with capture antibody overnight at room temperature followed by blocking with Reagent Diluent (DuoSet ELISA Reagent Kit) for 2 h. One hundred microliters cell culture media, or diluted fetal or maternal serum (1:40 in PBS), along with serial-diluted standards (0–4 ng/ml) were loaded to the plates and incubated overnight at 4 °C, followed by adding detection antibody and streptavidin-HRP subsequently. Optical density was determined using a microplate reader at 450 nm. The detection range of the assay is 0.0625 ng/ml to 4 ng/ml with intraplate coefficient of variation of the duplicates less than 10% and inter-plate coefficient of variation less than 15%.