Marsupials, like this red kangaroo, undergo a short gestational period compared to eutherian mammals.

Marsupials, like this red kangaroo, undergo a short gestational period compared to eutherian mammals. The corpus luteum isn’t maintained, and a placenta does not form. As a result, the gestational stage in red kangaroos, the largest extant marsupials, is only about 30 days

Source: http://news.nationalgeographic.com/news/2009/05/photogalleries/in-the-womb-animal-pictures/

Image:  Steven Downer via National Geographic