Final answer:
Ultrasounds are the safest imaging study to use during pregnancy because they do not use ionizing radiation. They are crucial for monitoring fetal development and helping physicians determine the optimal time for egg retrieval in fertility treatments. Progesterone is used post-retrieval to support the uterus for pregnancy.
Step-by-step explanation:
Out of the imaging studies listed for use during pregnancy, ultrasounds are considered to be the safest option. Unlike CT scans, PET scans, and X-rays, ultrasounds do not use ionizing radiation, which could potentially harm the fetus. Instead, ultrasounds use sound waves to create images of the baby in the womb, which is why they are commonly used throughout pregnancy to monitor the development of the fetus.
Blood tests and ultrasound scans are integral in monitoring the development of oocytes for retrieval during fertility treatments. They help determine the optimal time for egg retrieval, just before ovulation, ensuring the eggs are mature and ready for fertilization. After egg retrieval, progesterone is often administered to create an environment in the uterus conducive to supporting a pregnancy.