**Core Concept**
Radiation exposure during pregnancy can lead to fetal malformations, growth restriction, and even fetal death. Ionizing radiation, particularly in high doses, poses a significant risk to the developing fetus.

**Why the Correct Answer is Right**
The danger lies in the fact that ionizing radiation can cause DNA damage, leading to mutations in the developing fetus. This is because the fetus is most sensitive to radiation during organogenesis, which occurs between weeks 3 and 8 of gestation. The 10-week mark is a critical period, and exposure to ionizing radiation can lead to birth defects, such as microcephaly, cardiac anomalies, and limb abnormalities.

**Why Each Wrong Option is Incorrect**
**Option A:** Diagnostic ultrasound is a non-ionizing form of radiation, which is generally safe during pregnancy. It uses high-frequency sound waves to create images of the fetus and is commonly used for monitoring fetal development.

**Option B:** X-rays are a form of ionizing radiation, but they are typically used for diagnostic purposes in a controlled setting, such as in a hospital. The dose is usually low, and the risks are minimized when proper precautions are taken.

**Option C:** CT scans also involve ionizing radiation, but they are generally avoided during pregnancy unless absolutely necessary. The dose is higher than with X-rays, and the risks to the fetus are greater.

**Option D:** MRI (Magnetic Resonance Imaging) is a non-ionizing form of radiation, making it a safer option during pregnancy. It uses magnetic fields and radio waves to create images of the fetus and is often used for diagnostic purposes.

**Clinical Pearl / High-Yield Fact**
When performing imaging studies during pregnancy, it's essential to use the lowest possible dose of ionizing radiation and to choose non-ionizing modalities, such as ultrasound or MRI, whenever possible.

**Correct Answer:** C. CT scans