This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Ectopic Pregnancy (Hello ! Where are you ?), Subject name: Gynaecology & Obstetrics
Cause of vaginal bleeding in an ectopic pregnancy is shedding of the endometrium. This endometrium is known as: 
Select and explain. 
A. Decidua vera, B. Decidua basalis, C. Decidua capsularis, D. Decidua rubra
B: Decidua basalis
Answer: B: Decidua basalis 
Explanation: 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.