Definition: Term used to describe the formation of a fetus while another fetus is already present in the uterus. It occurs when eggs from two separate menstrual cycles are released, as opposed to normal dizygotic twins where multiple ova are expelled in a single cycle. Although common in animals, it is rare in humans, but can result in a twin or multiple pregnancy where the fetuses display a marked difference in gestational development.
In 2009, an Arkansas woman became pregnant with two babies due to superfetation. Ultrasound revealed that Julia Grovenburg
was pregnant with two babies conceived about two and a half weeks apart.