**Question:** If division of fertilized egg takes place in between 5-8 days, it would give rise to which kind of twin pregnancy? (c) March 2009, March 2013

**Core Concept:** Human fertilization and embryonic development

**Why the Correct Answer is Right:** The correct answer is D, Monochorionic-Diamniotic (MCDA) twin pregnancy. In this type of twin pregnancy, both fetuses develop from the same fertilized egg and share the same placenta, resulting in two amniotic sacs but two separate fetuses. The division of the fertilized egg occurs between the 5th and 8th days, which is consistent with MCDA twin pregnancies.

**Why Each Wrong Option is Incorrect:**

A. Monochorionic-Monoamniotic (MMA) twin pregnancy: In this type, both fetuses share the same amniotic sac but have separate placentas. Division of the fertilized egg occurs after 14 days, not between 5-8 days, making it incorrect.

B. Monochorionic-Dichorionic (MD) twin pregnancy: This type involves two fetuses, each with their own amniotic sac and placenta, but they share a single umbilical cord. Division of the fertilized egg occurs after 14 days, making it incorrect.

C. Monochorionic-Monoamniotic (MM) twin pregnancy: In this type, both fetuses share the same amniotic sac and placenta but have separate umbilical cords. Division of the fertilized egg occurs after 14 days, making it incorrect.

**Clinical Pearl:** Monochorionic-Diamniotic (MDD) twin pregnancies are the most common type of twin pregnancies, accounting for approximately 70-80% of all twin pregnancies. They occur when the fertilized egg divides after 14 days, resulting in two fetuses with separate amniotic sacs, placentas, and umbilical cords.

**Correct Answer Explanation:** Monochorionic-Diamniotic (MCDA) twin pregnancies are the result of the fertilized egg dividing between the 5th and 8th days, forming two fetuses with separate amniotic cavities, placentas, and umbilical cords. This type represents approximately 20-30% of all twin pregnancies. It is essential for medical students to understand the differences between these types of twin pregnancies as it influences the management and prognosis of the twin pregnancies.