Or try the primary literature: http://www.ncbi.nlm.nih.gov/pubmed/19360448
When focused with sufficient intensity, ultrasound causes powerful upwellings inside fluid mediums. These upwellings can cause gases to nucleate, further creating small microbubbles, called cavitation bubbles.
With continued exposure, these bubbles will gather gas until they reach a critical level where their structure cannot support the amount of pressure around them and so they implode.
The implosion creates a superheated jet of gas reaching 10,000 degrees Celsius and a shockwave of 300 times atmospheric pressure. It destroys fat if aimed at fat.
It also occurs in submarine propellers from them spinning under water. The picture shown is of cavitation damage to metal. Very destructive force.
This can also be used to treat cancers. The possibility exists also to create lesions in beneficial places, such as in the treatment of Parkinson’s disease with US (which will have to wait for another post). Kind of like internal acupuncture.
Cavitation is not a very common event in the fetus at lower levels of ultrasound exposure but the threat has been noted in literature. Keeping ultrasound as low as possible would prevent against it. However, higher intensities allowed in fetal imaging are potentially cavitating.