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).
Decidua is the term for the uterine lining (endometrium) during a pregnancy. It is formed under the influence of progesterone and serves to support and interact with the gestation. The decidua represents the maternal portion of the placenta.