Highlights•We quantify changes in decidual immune cell populations from 6 to 20 weeks of gestation.•CD56+ dNK cells increase in heterogeneity in second trimester decidua.•Decidual macrophage differentiated into M2 macrophage in second trimester.•Both CD4 and CD8 T cells increase in the second trimester decidua.•Dendritic cells remain in an immature state throughout first and second trimesters.