|Diagrammatic sections of the uterine mucous membrane: A. The non-pregnant uterus. B. Decidua parietalis; the mucus membrane in the pregnant uterus and not beneath the placenta.|
|Sectional plan of the gravid uterus in the third and fourth month.|
|Gray's||subject #12 59|
Decidua is the term for the uterine lining (endometrium) during a pregnancy, which forms the maternal part of the placenta. It is formed under the influence of progesterone and forms highly-characteristic cells.
The word comes from the Latin deciduus, meaning falling off or shedding.
After ovulation, in mammals, the endometrial lining becomes transformed into a secretory lining in preparation of accepting the embryo. Without implantation, the secretory lining will be absorbed (estrous cycle) or shed (menstrual cycle).
With implantation the lining now termed decidua evolves further during the pregnancy.
The decidua is shed during the parturition process.
Different layers of the deciduas have been described:
- a compact outer layer (stratum compactum)
- an intermediate layer (stratum spongiosum)
- a boundary layer adjacent to the myometrium ('Stratum Basalis').
That part of the decidua that interacts with the trophoblast is the decidua basalis (also called decidua placentalis). The remainder of the decidua is termed the decidua parietalis or decidua vera. Also, there is the decidua capsularis, which grows over the embryo on the luminal side, enclosing it into the endometrium and surrounding the embryo together with decidua basalis.
The decidua has a histologically-distinct appearance, displaying large polygonal decidual cells in the stroma. These are enlarged endometrial stromal cells, which resemble epithelium (and are referred to as "epithelioid").
Decidualization includes the process of differentiation of the spindle-shape stromal fibroblasts into the plump secretory decidual cells, which create a pericellular extracellular matrix rich in fibronectin and laminin (similar to epithelial cells).
Vascularity, as well as vascular permeability, is enhanced in the decidualizing endometrium.
Its leukocyte population is distinct, with the presence of large endometrial granular leukocytes being predominant, while polynuclear leukocytes and B-cells are scant.
The large granular lymphocytes (CD56 bright) are called "uterine NK cells" or "uNK cells" in mice, and "decidual NK cells" or "dNK cells" in humans.
As the maternal interface to the embryo the decidua participates in the exchanges of nutrition, gas, and waste with the gestation. It also protects the pregnancy from the maternal immune system. Further, the decidua has to allow a very controlled invasion of the trophoblast.
In invasive placental disorders like placenta accreta decidualization have been consistently found to be deficient.
Hormone production 
- In case of an extrauterine pregnancy, the endometrium nevertheless becomes decidualized. A woman may shed the lining in the form of a decidual cast, which may be mistaken as a miscarriage, when, in fact, the ectopic still persists.
- A decidual reaction can be observed in tissue of the peritoneum and ovary during a pregnancy, and represents a response of stromal tissue to progesterone.
- Decidua in a lymph node may mimic metastatic carcinoma.
Additional images 
- Williams obstetrics. (22nd ed. ed.). New York ; Toronto: McGraw-Hill Professional. 2005. ISBN 9780071413152.
- Wu, DC.; Hirschowitz, S.; Natarajan, S. (May 2005). "Ectopic decidua of pelvic lymph nodes: a potential diagnostic pitfall.". Arch Pathol Lab Med 129 (5): e117–20. doi:10.1043/1543-2165(2005)129e117:EDOPLN2.0.CO;2. PMID 15859655.
- Edmondson, N.; Bocking, A.; Machin, G.; Rizek, R.; Watson, C.; Keating, S. "The prevalence of chronic deciduitis in cases of preterm labor without clinical chorioamnionitis.". Pediatr Dev Pathol 12 (1): 16–21. doi:10.2350/07-04-0270.1. PMID 18171100.