Decidua directly beneath blastocyst implantation becomes the deciduas basalis. The decidua capsularis overlies the enlarging blastocyst and initially separates the conceptus from the rest of the uterine cavity. The remainder of the uterus is lined by deciduas parietalis. By 14 to 16 weeks' gestation, the expanding sac has enlarged to completely fill the uterine cavity. The resulting apposition of the decidua capsularis and parietalis creates the decidua vera, and the uterine cavity is functionally obliterated.