What term refers to the portion of the endometrium on which the conceptus rests?

• A. Decidua Basalis
• B. Decidua Capsularis
• C. Decidua Parietalis
• D. Double Decidua Sign

Answer: (A)

During early pregnancy, the endometrium differentiates into decidual tissue, and the part that directly supports the implanted conceptus becomes the decidua basalis. This region lies between the embryo and the myometrium and forms the maternal portion of the placenta as the chorionic villi invade it. The decidua capsularis covers the gestational sac toward the uterine cavity, while the decidua parietalis lines the rest of the uterine cavity. The double decidua sign is a separate ultrasound feature describing two decidual layers around the sac, not the surface on which the conceptus rests. So, the portion of the endometrium on which the conceptus rests is the decidua basalis.