<qa>  What is the principal function of the decidua?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 . 