Can an abdominal ct scan detect early pregnancy?

Possibly yes but: You don't want a ct scan if your think you are pregnant. Modern us exam is much safer for the fetus and is more sensitive.

Possibly: by accident. Generally pelvic CT is avoided in a pregnant or suspected pregnant patient, because of direct radiation to the developing fetus. Even if that was not the case, ultrasound is far superior at detecting early pregnancy than CT would be.