Differentiated endometrium of pregnancy which is shed (like a deciduous tree sheds its leaves) at childbirth or miscarriage, as part of the afterbirth. During pregnancy it has important hormonal functions (see decidual cells).

Plump endometrial stromal cells, lying between the glands of the endometrium, formed under the prolonged influence of progesterone, especially with establishment of pregnancy. Constitutes the decidual reaction to form the decidua of pregnancy, and produce prolactin (important for regulating water entering the gestational sac) and relaxin, which keeps the myometrium quiet.

A transformation of the lining of the uterus (the endometrium) in pregnancy. Complete confluence of endometrial stromal cells (lying between the endometrial glands) occurs, caused by prolonged (14 days or more) exposure to progesterone or a progestogen; normally happens only with pregnancy.

A partial confluence of stromal cells of the endometrium (lying between the endometrial glands), caused by prolonged exposure (10 days or more) to progesterone or a progestogen.

Loose connecting-type ('connective') tissue that lies between glands of the endometrium. Contains stromal cells that eventually respond to progesterone by becoming plump to form a more or less continuous sheet towards the end of the menstrual cycle (the predecidual reaction) or, more completely, with successful implantation and pregnancy (the decidual reaction).

The outermost layer of trophoblast, produced by cytotrophoblast where there is direct contact with maternal decidua rather than blood. The EVT cells travel into the decidua, reacting with NK cells and invading maternal blood vessels feeding the placenta, softening the walls and replacing the lining with fetal tissue, a process called "conversion". Also known as interstitial or intermediate trophoblast.

White blood cells or lymphocytes that form part of the 'innate' immune system capable of recognising foreign cells and dissolving them (hence their name: natural killer cells). Found normally, however, in the endometrium of the late secretory phase and in the decidua of pregnancy, where they have more friendly purposes in forming the boundary between mother and fetus. Unlike T cells and B cells, most NK cells do not have 'immunological memory', whereby previous contact with a particular antigen causes a faster, more deadly immune response second time round.

A small sample (or biopsy) is taken of the lining of the uterus (the endometrium) just before a period is expected, aiming to show advanced stages of progesterone's effect on it, in the form of an adequate predecidual reaction. This a very sensitive test of the adequacy of the luteal phase. The biopsy can be combined with a laparoscopy or carried out in isolation as an office procedure. It's best to be sure that there is no possibility of pregnancy during the cycle the test is conducted, as occasionally the biopsy jeopardises an early pregnancy (namely an early implantation, before the pregnancy test or serum hCG has become positive).

(PRL) The hormones produced by the pituitary gland to stimulate the production of milk in the breasts. Produced in increasing amount during pregnancy, although milk secretion is postponed until levels of estrogen and progesterone fall after the baby is born. For more on prolactin in the animal kingdom, and its importance in being produced by decidual cells in pregnancy.

An insulin-like protein with quite different functions. As the name suggests, tissues soften, relax or give-way upon exposure to it. Produced, for example, in the wall of the preovulatory follicle before ovulation, by the decidua to calm the myometrium of the pregnant uterus, and by the corpus luteum and the placenta to soften the ligaments of the pelvis in preparation for labor and childbirth.