Other animals

Most animals that lay eggs, such as birds and reptiles, including most ovoviviparous species, have an oviduct instead of a uterus. However, recent research into the biology of the viviparous (not merely ovoviviparous) skink Trachylepis ivensi has revealed development of a very close analogue to eutherian mammalian placental development.

In monotremes, mammals which lay eggs, namely the platypus and the echidnas, either the term uterus or oviduct is used to describe the same organ, but the egg does not develop a placenta within the mother and thus does not receive further nourishment after formation and fertilization.

Marsupials have two uteri, each of which connect to a lateral vagina and which both use a third, middle "vagina" which functions as the birth canal. Marsupial embryos form a choriovitelline placenta (which can be thought of as something between a monotreme egg and a "true" placenta), in which the egg's yolk sac supplies a large part of the embryo's nutrition but also attaches to the uterine wall and takes nutrients from the mother's bloodstream. However, bandicoots also have a rudimentary chorioallantoic placenta, similar to those of placental mammals.

The fetus usually develops fully in placental mammals and only partially in marsupials including kangaroos and opossums. In marsupials the uterus forms as a duplex organ of two uteri. In monotremes (egg-laying mammals) such as the platypus, the uterus is duplex and rather than nurturing the embryo, secretes the shell around the egg. It is essentially identical with the shell gland of birds and reptiles, with which the uterus is homologous.

In mammals, the four main forms of the uterus are: duplex, bipartite, bicornuate and simplex.

Duplex

There are two wholly separate uteri, with one fallopian tube each. Found in marsupials (such as kangaroos, Tasmanian devils, opossums, etc.), rodents (such as mice, rats, and guinea pigs), and lagomorpha (rabbits and hares).

Bipartite

The two uteri are separate for most of their length, but share a single cervix. Found in ruminants (deer, moose, elk etc.), hyraxes, cats, and horses.

Bicornuate

The upper parts of the uterus remain separate, but the lower parts are fused into a single structure. Found in dogs, pigs, elephants, whales, dolphins, and tarsiers, and strepsirrhine primates among others.

Simplex

The entire uterus is fused into a single organ. Found in higher primates (including humans and chimpanzees). Occasionally, some individual females (including humans) may have a bicornuate uterus, a uterine malformation where the two parts of the uterus fail to fuse completely during fetal development.

Two uteri usually form initially in a female and usually male fetus, and in placental mammals they may partially or completely fuse into a single uterus depending on the species. In many species with two uteri, only one is functional. Humans and other higher primates such as chimpanzees, usually have a single completely fused uterus, although in some individuals the uteri may not have completely fused.