This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Twin Pregnancy, Molar Pregnancy, Gestational Trophoblastic disease and contraception in special situations (Sour Grapes!), Subject name: Gynaecology & Obstetrics
To say twin discordance the difference in the two twin's weight should be: 
Select and explain. 
A. 15% with the larger twin as index, B. 15% with the smaller twin as index, C. 20% with the larger twin as index, D. 25% with the smaller twin as index
C: 20% with the larger twin as index
Answer: C: 20% with the larger twin as index 
Explanation: Discordant twins: The donor twin should be > 20% smaller in weight than the recipient twin Abdominal circumference difference of >15%