## **Core Concept**
The determination of chorionicity in twin pregnancies is crucial for assessing the risk of complications and planning appropriate management. Chorionicity refers to the number of chorions present in a multiple pregnancy, which can be determined through ultrasound examination. The **chorion** is a vital membrane surrounding the fetus.

## **Why the Correct Answer is Right**
The best time for detecting chorionicity in a twin pregnancy on ultrasound (USG) is between **11-14 weeks** of gestation. During this period, the **lambda sign** (or the **thick** and **distinct** chorionic membrane separating the two amniotic sacs) and the **twin peak sign** are more easily visualized, providing clear evidence of dichorionicity or monochorionicity. Accurate determination of chorionicity helps in assessing the risk for complications such as twin-to-twin transfusion syndrome (TTTS) in monochorionic twins.

## **Why Each Wrong Option is Incorrect**
- **Option A:** Detection can be done earlier (as early as 6-10 weeks), but it's less accurate due to the smaller size of the gestational sacs and the difficulty in visualizing the chorionic membrane clearly.
- **Option B:** While detection can still be done after 14 weeks, the optimal time for clear visualization and accurate determination of chorionicity is before 14 weeks.
- **Option D:** This option suggests a much later gestation, which, although still possible, is not considered optimal for the initial assessment of chorionicity.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that **chorionicity should be determined in the first trimester**, ideally between 11-14 weeks. Early determination of chorionicity helps in risk stratification and planning for potential complications. A monochorionic pregnancy (sharing the same placenta) carries a higher risk of complications like TTTS compared to a dichorionic pregnancy (having two separate placentas).

## **Correct Answer:** . 11-14 weeks.