2.8. Immunohistochemistry

Human decidua and villi tissues were fixed with 4% paraformaldehyde for 12 h at room temperature. After embedding the tissues in paraffin, 4‐μm sections were prepared. Xylene and a graded series of ethanol were used to deparaffinise and rehydrate the tissue sections, respectively. The tissue sections were incubated with a rabbit monoclonal antibody against human GDF15 (1:100; ab206414, Abcam, Cambridge, UK) diluted in PBS supplemented with 1% bovine serum albumin (BSA) and 0.3% Triton X‐100 (Solarbio) at 4°C overnight, followed by incubation with biotinylated goat anti‐rabbit IgG. After washing with PBS, the sections were stained with the chromogenic reagent 3,3′‐diaminobenzidine (ZSGB‐BIO, Beijing, China) for 22–25 s, followed by staining with haematoxylin. Images were captured under a microscope (Olympus, Tokyo, Japan), and the protein signals were analysed using ImageJ software (National Institute of Health, Bethesda, MD, USA). Data were analysed using SPSS 25.0 (IBM, Armonk, NY, USA) and GraphPad Prism 9 (GraphPad, San Diego, CA, USA) software.