To date, a multiplicity of factors contributing to the establishment and progression of a successful pregnancy have been postulated. There is emerging evidence that decidual leukocytes could be decisive factors during pregnancy. Despite numerous investigations on immune cells in human early pregnancy decidua, little is known about the physiological composition and proportion of the various immune cell populations during the different phases of pregnancy. In this study, we therefore analyzed the proportion of the dominant decidual leukocytes in human tissue samples derived from all phases of pregnancy.

Single cell suspensions were prepared from decidual samples from 205 patients at 6–40 weeks of gestation. Cell populations were analyzed by flow cytometry, and immune cell populations were quantified as percentage of decidual CD45⁺ cells.

There was generally no difference in immune cell counts comparing decidua of healthy gestations and those with systemic inflammation. Overall, the proportion of uNK cells continuously decreased, while the amount of monocytes, immature dendritic cells, and T cells increased until term. Striking modifications in cell counts were seen during the 7th week compared with the 6th and later weeks of gestation.

Studying the proportion of decidual immune cells during pregnancy, we detected a unique pattern which could be useful to design novel therapies for pathological conditions during pregnancy.