**Core Concept**
The double decidual sac sign (DDSS) is a sonographic finding that reflects the presence of two separate decidual sacs in the uterine cavity, each containing a developing embryo. It is a key feature in early pregnancy assessment and is used to distinguish normal singleton gestation from other conditions.

**Why the Correct Answer is Right**
DDSS is observed in early normal singleton pregnancies, where two distinct decidual sacs are seen within the uterus, each associated with a developing embryo. This sign is typically seen before the gestational sacs become visible as a single gestational sac. It is a hallmark of a normal intrauterine pregnancy and helps differentiate it from other conditions like ectopic pregnancy or multiple gestations.

**Why Each Wrong Option is Incorrect**
Option A: Monoamniotic dichorionic twins do not exhibit DDSS; instead, they show a single gestational sac with two embryos in a shared amniotic cavity. DDSS is not seen in monoamniotic pregnancies.
Option B: Diamniotic dichorionic twins show two separate gestational sacs, but these are typically described as "two gestational sacs" rather than "double decidual sacs." DDSS is not the defining sign for this type.
Option C: Ectopic pregnancy lacks a uterine decidual sac and shows no intrauterine gestational sac, so DDSS is absent and not diagnostic.

**Clinical Pearl / High-Yield Fact**
DDSS is most reliably seen in early normal singleton pregnancies and is a key ultrasound feature to rule out ectopic pregnancy or abnormal gestation. It is not seen in multiple gestations or ectopic pregnancies and should not be mistaken for signs of twins.

✓ Correct Answer: D. Normal pregnancy