It is generally believed that diseases of the decidua are common causes of premature discharge of the ovum, but little is known as to their nature. Atrophy and hypertrophy are stated to occur, the latter perhaps as the result of inflammation.
Inflammation of the decidua (Deciduitis or Endometritis decidualis) apparently leads to various local thickenings and irregularities of the decidua. It occurs when, before pregnancy, there has been a chronic inflammation of the mucous membrane of the uterus. The changes are observed in the membrane discharged along with aborted foetus.
Haemorrhage from the decidua is frequent in the course of pregnancy, and may be the cause of abortion.
In some cases the foetus is retained and the blood may accumulate in the chorion and the surface of the amnion. The blood coagulates, and, the fibrine condensing the structures, a fleshy mass is produced, in the midst of which the dead foetus may be no longer discoverable. This mass, may remain long in the uterus, to be discharged afterwards as a fleshy mole. Sometimes it is retained so long as to become infiltrated with lime salts, a stony mole being ultimately discharged.