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
If division of feilized egg takes place in between 5-8 days. It would give rise to which kind of twin pregnancy: March 2009, March 2013 (c) 
Select and explain. 
A. Monochorionic/monoamniotic, B. Monochorionic/diamniotic, C. Dichorionic/ monoamniotic, D. Siamese twins
B: Monochorionic/diamniotic
Answer: B: Monochorionic/diamniotic 
Explanation: Ans. B: Monochorionic/diamniotic Approximately 70% of monozygotic twins are monochorionic /diamniotic. If splitting occurs later (ie, 9-12 d after feilization), monochorionic/ monoamniotic placentation occurs. Monochorionic/monoamniotic twins are rare; only 1% of monozygotic twins have this form of placentation. twins have a common placenta, with vascular communications between the 2 circulations. These twins can develop twin-to-twin transfusion syndrome (TTTS). If twinning occurs more than 12 days after feilization, then the monozygotic feilized ovum only paially splits resulting in conjoined twins.