**Core Concept**
The intrauterine death (IUD) in a pregnant woman can be demonstrated on X-ray by the presence of calcifications in the fetal skeleton, which can be seen as early as 28 weeks of gestation. This is due to the process of calcification, where the fetal bones start to ossify and become visible on X-ray.

**Why the Correct Answer is Right**
Calcification of fetal bones is an early indicator of IUD on X-ray. The bones that are most commonly involved in calcification are the skull, vertebrae, and long bones. The calcification process starts around 20-24 weeks of gestation, but it becomes more prominent and visible on X-ray around 28 weeks. This is because the amount of calcification increases as the bones continue to ossify and mature.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the presence of an intrauterine gas bubble is not an early indicator of IUD on X-ray. Gas bubbles can be seen in normal pregnancies, especially after amniocentesis or other invasive procedures.

**Option B:** This option is incorrect because the presence of an intrauterine hemorrhage is not an early indicator of IUD on X-ray. Hemorrhage can be seen in various conditions, including normal pregnancies, and is not specific to IUD.

**Option C:** This option is incorrect because the presence of an intrauterine mass is not an early indicator of IUD on X-ray. Masses can be seen in various conditions, including normal pregnancies, and are not specific to IUD.

**Clinical Pearl / High-Yield Fact**
One important point to remember is that the presence of calcifications in the fetal skeleton on X-ray is a strong indicator of IUD, especially if the calcifications are seen in the absence of fetal movement or other signs of life.

**Correct Answer: C. Calcification of fetal bones.**