**Core Concept**
Anti-D immunoglobulin is used to prevent the formation of antibodies against Rh-positive red blood cells in Rh-negative women. This is crucial to prevent hemolytic disease of the newborn (HDN) in subsequent pregnancies.

**Why the Correct Answer is Right**
Anti-D immunoglobulin works by coating the surface of any Rh-positive red blood cells entering the maternal circulation, marking them for destruction by the maternal immune system. This prevents the formation of anti-Rh antibodies, which can cross the placenta and cause HDN in Rh-positive fetuses. The administration of anti-D immunoglobulin is typically recommended after delivery, abortion, or miscarriage involving an Rh-positive fetus.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect as anti-D immunoglobulin is not typically indicated in cases of ectopic pregnancy, where the risk of fetal-maternal hemorrhage is low.

**Option B:** This option is incorrect as anti-D immunoglobulin is not typically indicated in cases of spontaneous abortion before 12 weeks of gestation, where the risk of fetal-maternal hemorrhage is low.

**Option C:** This option is incorrect as anti-D immunoglobulin is not typically indicated in cases of Rh-negative women who are not pregnant, as there is no risk of HDN.

**Clinical Pearl / High-Yield Fact**
Anti-D immunoglobulin should be administered within 72 hours of delivery, abortion, or miscarriage to prevent the formation of anti-Rh antibodies.

**Correct Answer:** D. In cases of postpartum hemorrhage involving an Rh-positive fetus.