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Part of uterus modified in pregnancy

The decidua is the modified mucosal lining of the uterus (that is, modified endometrium) that forms every month, in preparation for pregnancy. It is shed off each month when there is no fertilised egg to support. The decidua is under the influence of progesterone. Endometrial cells become highly characteristic. The decidua forms the maternal part of the placenta and remains for the duration of the pregnancy. After birth the decidua is shed together with the placenta. 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 natural killer cells| (uNK cells).

After ovulation, in placental mammals, the endometrial lining becomes hypertrophic and vascular under the influence of the sex hormones, estrogen and progesterone. In animals exhibiting hemochorial placentation, the endometrium undergoes decidualization following implantation. If implantation does not occur, the secretory lining will be absorbed (estrous cycle) or shed (menstrual cycle). The decidua is shed with the placenta during birth.

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.

The decidua secretes hormones, growth factors, and cytokines. It has receptors for estrogen, progesterone, growth hormone, and others. Among its products are hormones commonly associated with other organs such as cortisol, CRF, GnRH, prolactin, and relaxin. Decidual prolactin is not under dopaminergic control. Insulin like growth factor binding protein 1 (IGFBP1) also called placental protein 12, and PAEP (glycodelin) appear to be specific products of the secretory and decidual lining. Other factors released include interleukin 15 and vascular endothelial growth factor (VEGF). A reasonable understanding of the role and interplay of these hormones and factors has not been evolved.

In case of an ectopic 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 pregnancy 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.

A long lasting infection of the decidua, chronic deciduitis, is associated with pre term labour.

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