**Core Concept**
During pregnancy, certain medical investigations must be avoided due to potential harm to the fetus or the mother. The choice of diagnostic test depends on the clinical context and the gestational age of the fetus.

**Why the Correct Answer is Right**
The correct answer is based on the principle of avoiding ionizing radiation during pregnancy, as it can increase the risk of congenital abnormalities and teratogenic effects. The use of ionizing radiation, such as in CT scans or conventional radiography, can lead to the production of free radicals that may damage fetal DNA and disrupt normal development.

**Why Each Wrong Option is Incorrect**
**Option A:** Magnetic Resonance Imaging (MRI) is generally considered safe during pregnancy, as it does not involve ionizing radiation. However, it's still essential to use MRI cautiously and only when absolutely necessary, as it may not be available in all settings.
**Option B:** Ultrasound is a widely used imaging modality during pregnancy, as it does not involve ionizing radiation and is generally considered safe for both the mother and the fetus.
**Option C:** Echocardiography is a non-invasive, radiation-free imaging technique that uses ultrasound waves to create images of the heart. It is commonly used during pregnancy to assess fetal cardiac function.

**Clinical Pearl / High-Yield Fact**
When ordering imaging studies during pregnancy, it's essential to consider the gestational age and the clinical indication for the test. The American College of Radiology (ACR) and the Society of Radiologists in Ultrasound (SRU) provide guidelines for safe imaging practices during pregnancy.

**Correct Answer: B. Ultrasound.**