The decidua is the changed endometrium (the lining of the uterus) after the blastocyst (fertilized ovum after 4 - 9 days of development) is implanted onto the endometrium. At this stage, there are 3 regions of decidua: decidua basalis: between the blastocyst and the uterine muscular wall, decidua capsularis: covers the blastocyst; decidua parietalis: the rest of the lining of the uterus.