Superfetation

Superfetation (also spelled superfoetation – see fetus) is the simultaneous occurrence of more than one stage of developing offspring in the same animal. In mammals, it manifests as the formation of an embryo from a different estrous cycle while another embryo or fetus is already present in the uterus. When two separate instances of fertilisation occur during the same menstrual cycle, it is known as superfecundation.

Superfetation is claimed to be common in some species of animals, but is extremely rare in humans. In mammals, it can occur only where there are two uteri, or where the estrous cycle continues through pregnancy. The risk with superfetation in humans is that the second baby is often born prematurely, which can increase its odds of experiencing lung development problems.

In animals

Animals that have been claimed to be subject to superfetation include rodents (mice and rats), rabbits, farm animals (horses and sheep), marsupials (kangaroos and sugar gliders), and primates (humans). Superfetation has also been clearly demonstrated and is normal for some species of poeciliid fishes.