This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Gynaecology & Obstetrics
Gestational sac diameter of ______ is suggestive of anembryonic pregnancy, if embryo is not visible 
Select and explain. 
A. 5 mm, B. 10 mm, C. 15 mm, D. 20 mm
D: 20 mm
Answer: D: 20 mm 
Explanation: Embryo should be visible transvaginally once the mean sac diameter has reached 20 mm, otherwise the gestation is anembryonic.