Final answer:
The definitive diagnostic modality for abdominal aortic aneurysm is a CT scan due to its ability to create detailed images. However, during pregnancy, ultrasonography is the safest imaging study for any condition including checking fetal development.
Step-by-step explanation:
The definitive diagnostic modality for an abdominal aortic aneurysm (AAA) is a CT scan. This imaging technique constructs detailed X-ray images which are essential for evaluating the presence, size, and extent of an AAA. During pregnancy, however, the Safest imaging study to use is ultrasonography, as it is the least invasive and uses no ionizing radiation, making it more suitable in sensitive situations like pregnancy. An Ultrasound can detect various conditions, but for AAA specifically, a CT scan provides the most detailed information.