Because of its complicated reproductive set-up, female kangaroos can be perpetually pregnant. While one joey is developing inside the pouch, another embryo is held in reserve in a uterus, waiting for its sibling to grow up and leave. So, basically, kangaroos can be pregnant forever.
However, because the anatomy of a marsupial doesn’t allow for a large fetus to pass through it, the joeys are so tiny when they’re born (about the size of a jellybean when it leaves the vagina) and start climbing to the pouch, where they grab on to nurse for the next few months of development.
