Ultrasound works on sound waves and no radiation is used.  That is one of the reasons that it is used in pregnancy.  It does not cause harm to the foetus and is quite accurate to assess foetal aging and maturity. It often excludes major foetal abnormalities.

Ultrasound is used to assess soft tissue structures of the body. This includes abdomen, chest, muscles and tendons.

Ultrasound cannot be used on bony, or gas filled structures.

Gallbladder to exclude gall-stones.
Liver, Pancreas and Spleen to exclude pathology.
Kidneys and bladder to exclude kidney stones, masses and obstructive lesions.

Includes flow studies and anatomy in arteries and veins. Also used to exclude DVT or deep vein thrombosis.