**Core Concept**
Antepartum hemorrhage (APH) refers to vaginal bleeding that occurs after 28 weeks of gestation, regardless of its severity, and before the birth of the fetus. This condition can be caused by various factors, including placental abruption, placenta previa, and cervical or uterine abnormalities.

**Why the Correct Answer is Right**
The American College of Obstetricians and Gynecologists (ACOG) defines antepartum hemorrhage as bleeding that occurs after 28 weeks of gestation. This is because bleeding before 28 weeks is often associated with different etiologies and management strategies. The 28-week cutoff is a critical milestone in pregnancy, as it marks the transition from the second to the third trimester. After 28 weeks, the risk of bleeding increases due to the growing size of the placenta and the uterine distension.

**Why Each Wrong Option is Incorrect**
**Option A:** Bleeding at 12 weeks is typically associated with a miscarriage or an ectopic pregnancy, which are not considered antepartum hemorrhages.

**Option B:** Bleeding at 18 weeks may be related to a molar pregnancy or a cervical insufficiency, but it is not classified as antepartum hemorrhage.

**Option C:** Bleeding at 20 weeks can be caused by various factors, including cervical or uterine abnormalities, but it is not the standard definition of antepartum hemorrhage.

**Clinical Pearl / High-Yield Fact**
It is essential to remember that antepartum hemorrhage can occur at any gestational age, but the severity and management of the condition may differ depending on the gestational age. A thorough evaluation and management plan should be implemented promptly to prevent maternal and fetal complications.

**✓ Correct Answer: D. 28 weeks**