**Core Concept**
Polyhydramnios is a condition characterized by an excessive accumulation of amniotic fluid, which can lead to potential complications in pregnancy. The normal volume of amniotic fluid is approximately 500-800 mL. Polyhydramnios is typically defined as an amniotic fluid index (AFI) greater than 25 cm.

**Why the Correct Answer is Right**
The correct answer is based on the definition of polyhydramnios. Amniotic fluid volume in polyhydramnios is indeed more than the normal range. This condition can occur due to various factors, including fetal anomalies, maternal diabetes, and placental insufficiency. The increased amniotic fluid volume can be attributed to an imbalance between fetal urine production and reabsorption, as well as an excessive amount of fetal swallowing and breathing movements.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is likely incorrect as it is below the normal range of amniotic fluid volume. Polyhydramnios is characterized by an excessive accumulation of amniotic fluid, not a decrease.

**Option C:** This option is also incorrect as it is within the normal range of amniotic fluid volume. Polyhydramnios is defined as an amniotic fluid index greater than 25 cm, which is above this value.

**Clinical Pearl / High-Yield Fact**
A key aspect to remember is that polyhydramnios can be associated with fetal anomalies, such as esophageal atresia or duodenal atresia, which can lead to an imbalance between fetal urine production and reabsorption. Additionally, maternal diabetes can also contribute to the development of polyhydramnios.

**Correct Answer:** D