 Immune function in pregnancy From initial implantation of the conceptus, the maternal uterine endometrium undergoes decidualisation to support placental development and function. The resulting decidua is a tissue formed from the maternal endometrium, originating from epithelial and stromal cells, and is characterised by invasion from the extraembryonic fetal-derived trophoblasts and close cell-cell juxtaposition of these different tissues The principal function of the decidua is to facilitate early fetal-maternal exchange of nutrients, gases and waste, while also acting as a secretory source of steroid hormones, cytokines and growth factors 9.However, the decidua also plays a key role in protecting pregnancy against maternal immune surveillance10 . 