Swamp wallaby is a marsupial (young are carried in a pouch) related to the kangaroo, is pregnant throughout its adult life.
It typically conceives a new embryo day before delivering the new-born from its previous pregnancy.
Female wallabies have two uteri and two separate ovaries. At the end of a pregnancy in one uterus, a new embryo develops in the other uterus.
Wallabies regularly have an embryo in the uterus, a young joey in the pouch, and some third semi-dependent young at foot, still drinking its mother’s milk.