**Core Concept**
Fetal blood loss occurs due to the rupture of fetal blood vessels, which can be caused by various factors during pregnancy, labor, or delivery. This can lead to fetal anemia, hypoxia, and even fetal demise. The placenta, umbilical cord, and fetal vessels are the primary sites where blood loss can occur.

**Why the Correct Answer is Right**
The most common cause of fetal blood loss is the rupture of the umbilical cord vessels. This can happen due to mechanical injury during delivery, umbilical cord prolapse, or vasa previa, a condition where fetal blood vessels are exposed in the birth canal. The fetal vessels are prone to rupture due to their thin walls and the high pressure of fetal blood flow.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect as it does not specify a condition that directly leads to fetal blood loss. Various maternal conditions can cause fetal distress, but they do not directly result in fetal blood loss.
**Option B:** This option is incorrect as it refers to a condition that affects the mother, not the fetus. Maternal hypotension can affect fetal well-being, but it does not directly cause fetal blood loss.

**Clinical Pearl / High-Yield Fact**
Fetal blood loss can be a catastrophic event, and prompt identification and management are crucial to prevent fetal demise. A high index of suspicion, especially in cases of vaginal bleeding or sudden fetal distress, is essential for timely intervention.

**Correct Answer:** D.