=Decidua Vera.=—The part of the decidua vera to which the ovum becomes attached and where the placenta afterward forms is called the decidua serotina. The presence of the ovum acts as an irritant to the spot where it lodges, and active growth takes place. At the same time the ovum burrows into the mucosa and gradually becomes embedded. The vera closes over the ovum. The decidua vera is a mold of the interior of the uterus. The external surface is rough where it has been torn from the underlying tissues of the uterus, and the internal aspect which is in contact with the ovum is smooth. It is thickest at the third month; after this time the membrane becomes thinner and thinner, until by the end of pregnancy it is very thin. It is to a considerable extent expelled at labor and forms part of the membranes. =Ovum.=—As the ovum develops the decidua reflexa expands until it fills the decidua vera at the end of the second month, and by the end of the third month it has intimately united with the decidua vera to form one membrane. Until these two membranes have coalesced the ovum is not free in the uterine cavity, but is enclosed in the decidua reflexa. =Pregnant Uterus.=—As pregnancy proceeds the uterus ceases to be a pelvic organ; it becomes rounded, and after the sixth month gradually assumes an ovoid shape. At the end of the third month it has reached to the brim of the pelvis. By the sixth month it is level with the umbilicus, and by the ninth it has reached to the tip of the sternum. During the last fortnight the organ sinks a little, and this relieves to some extent the shortness of breath which has been caused by the pressure on the diaphragm.