**Core Concept**
Placenta Praevia is a condition in pregnancy where the placenta partially or completely covers the internal cervical os, leading to vaginal bleeding. This condition is often associated with bleeding in the second or third trimester.

**Why the Correct Answer is Right**
The correct answer is related to the pathophysiology and clinical presentation of Placenta Praevia. In this condition, the placenta covers the internal cervical os, causing bleeding due to the separation of the placenta from the uterus. The bleeding can be heavy and is often associated with pain and discomfort. The risk factors for Placenta Praevia include previous uterine surgery, multiple gestations, and placenta previa in a previous pregnancy.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because Placenta Praevia is not typically associated with fetal distress. Fetal distress is more commonly seen in conditions such as placental abruption or umbilical cord prolapse.
* **Option B:** This option is incorrect because Placenta Praevia is often diagnosed with ultrasound, which can demonstrate the placenta covering the internal cervical os.
* **Option C:** This option is incorrect because Placenta Praevia is not typically associated with polyhydramnios. Polyhydramnios is a condition where there is an excessive amount of amniotic fluid, which is more commonly seen in conditions such as maternal diabetes or fetal anomalies.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that Placenta Praevia is a condition that requires immediate attention, as it can lead to significant maternal and fetal morbidity if not managed promptly.

**Correct Answer: D. Fetal distress**