Superfetation is the formation of a fetus while another fetus is already present in the uterus.
When there are two separate instances of fertilisation during the same cycle, rather than different cycles, it is known as superfecundation.
Superfetation is said to be common in several species of animals, but is extremely rare in humans.
It can occur only when the menstrual cycle continues through pregnancy.
The most recent reported case involves Todd and Julia Grovenburg of Fort Smith, Arkansas.