Monoamniotic twins are identical or semi-identical twins that share the same amniotic sac within their mother's uterus.[1] Monoamniotic twins are always monochorionic and are usually termed Monoamniotic-Monochorionic ("MoMo" or "Mono Mono") twins.[1][2] They share the placenta, but have two separate umbilical cords. Monoamniotic twins develop when an embryo does not split until after formation of the amniotic sac,[1] at about 9–13 days after fertilization.[3] Monoamniotic triplets or other monoamniotic multiples[4] are possible, but extremely rare.[1] Other obscure possibilities include multiples sets where monoamniotic twins are part of a larger gestation such as triplets, quadruplets, or more.