A lady from Arkansas got pregnant twice, nothing unusual about that. What is so unusual is that she got pregnant twice at the same time. In other words she got pregnant while she was already pregnant. The medical term for this is "superfetation" meaning conceiving while pregnant. There are only about ten documented human cases of superfetation, and there is some question to if some of them were legitimate / misdiagnosed.
"Doctors successfully located Todd and Julia Grovenburg's growing baby girl Jillian, but then discovered another smaller baby -- what could be Jillian's younger brother -- growing beside her.
The Grovenburgs may have conceived their son Hudson a full two-and-a-half weeks after Jillian, according to statements given to KFSM-TV in Ft. Smith-Fayetteville, Ark.
Different from identical twins or fraternal twins, the Grovenburg babies would have separate due dates and are considered to be a rare medical occurrence.
Reports of superfetation, or conceiving while pregnant, are so rare that Boyle said, "There is no prevalence or incidence in the literature. I could only find about 10 reported cases."
Grovenburg's obstetrician, Dr. Michel Muylaert, confirmed to KFSM-TV that the Grovenburgs may be dealing with the extraordinarily rare case of superfetation."