I had to look it up too. You might want to stop reading after the Kangaroo paragraph.
Kangaroos are born after less than a month, and then climb into the pouch, attach to a teat and stay there for half a year before being ready to face the outside world.
Spotted Hyenas gestate for about 110 days and then have to make it though the mother's pseudo penis, which can tear with first-time mothers (fatal to her) and can suffocate the cub (fatal to them) with cubs of first-time mums only having a 40% chance of surviving birth.