A Colombian woman has given birth to a baby girl who was found to have her twin living inside her abdomen.

The case is believed to be a rare instance of “fetus-in-fetu”. While the condition is not completely unheard of, there have been fewer than 100 cases reported in the medical literature.

Itzamara was recently born, alongside her fetal twin, in the Colombian seaport of Barranquilla. As reported by local newspaper El Heraldo, doctors noticed the abnormality while Itzamara was still in the womb of her mother, Monica Vega, which is actually pretty unusual as it’s typically only noticed in infancy. In one exceptional case, doctors diagnosed a 47-year-old man with fetus-in-fetu.

During an ultrasound in week 35 of the pregnancy, they noticed the baby appeared to have a cyst, but a closer inspection revealed it was actually a "fetus" that was inside the abdomen.

“[It’s] one of the strangest and most fascinating things that are seen in maternal-fetal medicine,” Dr Miguel Antonio Parra Saavedra, who dealt with the case at the La Merced clinic in Barranquilla, told El Heraldo.

Itzamara was born via Cesarian-section at 37 weeks on February 22 with a healthy birth weight of 3.2 kilograms (7 pounds). Just one day after her birth, doctors had to perform keyhole surgery to remove the barely formed – but still growing – twin.

"We communicated the scenario to the mother, since it is a supremely rare event, but she had never heard it before. In fact, most people do not know that this phenomenon can occur in human nature," added Dr Parra Saavedra.

The fetal twin was just 14 millimeters tall with no heart, no brain, and only rudimental limbs. After cutting the umbilical cord that bound them, the fetus died.

Scientists are not certain why fetus-in-fetu can arise; however, it’s believed to occur during the first few weeks of twin pregnancies after one fetus wraps around and envelops the other. The enveloped twin then becomes “parasitic” and relies on the blood supply of the “host” twin. The condition is sometimes misdiagnosed as a teratoma. This is a germ cell tumor made up of several different types of tissue, such as hair, muscle, nail, or bone. A highly developed teratoma can, therefore, sometimes appear to look like an underdeveloped fetus. 

As for Itzamara, she is in good health and well on her way towards making a full recovery.

“She has a little scar on her abdomen, but she is a normal baby now except that the whole world is talking about her,” Dr Parra-Saavedra told The New York Times.