**Core Concept**
Intrauterine Growth Restriction (IUGR) refers to a condition where a fetus does not grow at a normal rate inside the womb. This can be caused by various factors, including maternal, placental, or fetal issues. IUGR is a significant concern as it can lead to preterm birth, low birth weight, and increased risk of perinatal mortality.

**Why the Correct Answer is Right**
IUGR is commonly seen in cases of **placental insufficiency**. The placenta plays a crucial role in providing oxygen and nutrients to the fetus. When the placenta is not functioning properly, it can lead to a decrease in fetal growth. This can be due to various factors such as **pre-existing hypertension**, **preeclampsia**, or **uteroplacental ischemia**. The placenta's insufficient function can lead to a decrease in fetal growth, resulting in IUGR.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect as IUGR is not typically associated with **multiple gestations**, where the presence of multiple fetuses can actually lead to a higher birth weight.

**Option B:** This option is incorrect as **maternal obesity** is not a direct cause of IUGR. While maternal obesity can lead to various pregnancy complications, it is not a primary risk factor for IUGR.

**Option C:** This option is incorrect as **maternal diabetes** can actually lead to **fetal macrosomia**, where the fetus grows larger than normal, rather than IUGR.

**Clinical Pearl / High-Yield Fact**
A key risk factor for IUGR is **maternal hypertension**, particularly in cases where hypertension develops after 20 weeks of gestation. This is because hypertension can lead to placental insufficiency, resulting in IUGR.

**Correct Answer:** D.