Final answer:
Ultrasounds are the safest imaging study to use during pregnancy as they do not expose the fetus to radiation.
Step-by-step explanation:
The most safely used imaging study during pregnancy is ultrasounds. Ultrasounds use sound waves to create images of the body and do not expose the fetus to any harmful radiation. CT scans, PET scans, and X-rays all involve the use of ionizing radiation, which may pose a risk to the developing fetus.