Final answer:
Ultrasound is the safest imaging study that can be used during pregnancy, as it uses sound waves instead of ionizing radiation and can monitor fetal development without risk to the fetus.
Step-by-step explanation:
In the context of prenatal testing options, one of the imaging studies that can be used most safely during pregnancy is ultrasound. Unlike CT scans, PET scans, or X-rays, ultrasounds do not use ionizing radiation, which can be harmful to the developing fetus. Instead, ultrasounds use sound waves to create images of the baby, which are considered safe at any stage of pregnancy. Ultrasounds are commonly used not only to monitor the baby's development but also to diagnose potential issues early on.
Prenatal genetic diagnosis (PGD) represents another facet of prenatal testing, where genetic screening of embryos is conducted during in vitro fertilization before implantation. While PGD is not an imaging study, it can provide early detection of genetic disorders. However, the earliest non-invasive test specifically for prenatal care would still be the ultrasound.