In one sense, that's perfectly obvious, what with the womb being fluid-filled. Still, there's equal parts hypnosis and cute in actually watching unborn wallabies paddling.
The ABC reports on the research which covered the incredibly short 26 day pregnancy cycle of the Tamar Wallaby, including the last eight days when the cells of the baby Wallaby are identifiable at all. The reason why it's swimming is essentially so that it's ready to head from the womb to its mother's pouch. All I know is that I find it strangely hypnotic to watch.
Wallabies do 'Australian crawl' in the womb [ABC]