**Core Concept**
Multiple pregnancies, including twins, triplets, and other multifetal gestations, can be classified based on chorionicity, which refers to the number of placentas present. Chorionicity is crucial in determining the risk of complications such as twin-to-twin transfusion syndrome (TTTS) and selective intrauterine growth restriction (sIUGR).

**Why the Correct Answer is Right**
The Twin Peak Sign is a sonographic marker used to determine chorionicity in twin pregnancies. It is characterized by two triangular areas of echogenicity (the "twin peaks") at the base of the intertwin membrane, which represents the insertion of the intertwin membrane into the decidua basalis. This sign is specific to dichorionic twin pregnancies, where each fetus has its own separate placenta. The presence of the Twin Peak Sign indicates that the twins are dichorionic, which is associated with a lower risk of complications compared to monochorionic twins.

**Why Each Wrong Option is Incorrect**
**Option A:** Fetuses of the same gender do not exclude dichorionicity, as dichorionic twins can be the same sex. In fact, the majority of dichorionic twins are the same sex.

**Option C:** A thick separating membrane is actually a feature of dichorionic twins, not monochorionic twins. In dichorionic twins, the intertwin membrane is thicker and more echogenic due to the presence of two separate placentas.

**Option D:** Chorionicity can be reliably detected earlier than 16 weeks of gestation, typically around 10-12 weeks, using ultrasound and specific sonographic markers such as the Twin Peak Sign.

**Clinical Pearl / High-Yield Fact**
When assessing a twin pregnancy, it is essential to determine chorionicity as early as possible, as this information will guide further management and risk assessment.

**✓ Correct Answer: B. Twin Peak Sign is seen in dichorionicity**