"A parasitic twin is an identical twin that fails to fully separate in development," said John Ruge, a paediatric neurosurgeon at the hospital.
"In other words, not another independent twin, but a twin that was dependent on her body system, such that Dominique's heart and lungs provided the nourishment."
Doctors did not give Dominique's family name.
Nancy Swabb, from Edgebrook, Illinois, has looked after Dominique since she arrived in the United States for the surgery.
"She's been a joy. We really enjoyed having her in our family," said Swabb.
"We send a lot of photos and updates and so we know that Dominique's family sees what she's doing and seeing that she has two new teeth and she's learning to wave and doing all sorts of special things."
Swabb said she did not want to think about saying goodbye to Dominique when she returns to her family in the Ivory Coast.
"She has touched our hearts. She's a member of our family," Swabb said. "She's pretty amazing."
- Reuters, AAP