Superfetation (also spelt 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 menstrual cycle while another embryo or fetus is already present in the uterus. When there are two separate instances of fertilisation during the same 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 menstrual cycle continues through pregnancy. The risk with superfetation is that the second baby is often born prematurely, which can increase its odds of experiencing lung development problems.