The newborn baby, Itzamara, underwent life-saving surgery in Barranquilla, Colombia to remove the foetus.
Her twin was growing inside her tummy in its own amniotic sac, with an umbilical cord, bones and arms and legs, according to Colombian TV channel, Los Informantes.
It's believed that cases such as these - called 'fetus-in-fetu' births, are incredibly rare. They were first described in 1808 but rarely seen since.
This particular case broke new ground because the mother's obstetrician, Dr Miguel Parra-Saavedra, noticed it on a scan at 35 weeks' gestation by using a 3D/4D ultrasound technique.
The team delivered baby Itzamara early via C-section at 37 weeks at La Merced clinic to prevent her twin growing and damaging her internal organs.
The following day, they performed keyhole surgery to remove her 45-millimeter, 14-gram twin, who died when its umbilical cord was cut. The baby had arms and legs but no heart and no brain.
Dr Parra-Saavedra said, Itzamara was actually "pregnant" with her sister.
Now, a month later, little Itzamara is recovering well, with barely any signs of damage or impact to her abdomen.
It was "one of the strangest and most fascinating things you can see in maternal-fetal medicine," Dr Parra-Saavedra told his local paper The Herald.