In an unusual case of "fetus in fetu", a baby born by cesarian section needed to have her own C-section just 24 hours later to remove the twin she had absorbed in the womb.

Fetus in fetu is what happens when twin embryos do not divide properly. Usually with twins, the division occurs in the first week of gestation. Fetus in fetu happens when the division occurs much later, after around 17 days, and one twin is “absorbed” into the other, more commonly known as a “parasitic twin”.

It’s a very rare condition – occurring in around one in 500,000 pregnancies – and is usually only identified after birth. What made this case so unusual is it was identified before the baby’s mother, Monica Vega, gave birth to her daughter Itzamara, which meant they could plan for it.

Dr Miguel Parra-Saavedra, a high-risk pregnancy specialist in Barranquilla, Colombia, oversaw the birth, the New York Times reports. He first saw Vega when she was 35 weeks pregnant after her obstetrician initially thought she had a liver cyst. Using high-tech scanning equipment Dr Parra-Saavedra could see instead that there were two umbilical cords, one connecting Itzamara to her mother and the other connected to a mass that had formed inside of her.

Parasitic twins are sometimes misdiagnosed as teratoma – a type of tumor that may contain bones, hair, teeth, or muscle tissue that people can live with for years before diagnosis. But Parra-Saavedra was confident in his findings that this was a parasitic twin that was being supported and nourished by its host, and would not survive on its own.

Worried that it may continue to grow and damage the baby's organs, or that she may be negatively impacted by sharing nutrients to support the two, they decided to deliver Itzamara right away at 37 weeks via C-section.

Just 24 hours after she was born, Itzamara – who weighed a healthy 3.2 kilograms (7 pounds) – underwent her own surgery to remove the “twin”. Luckily, Itzamara suffered no complications and is doing well.

“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 back in March, after the story was featured on the Colombian news program The Informants.

As for the parasitic twin they removed, the doctors confirmed it would not have survived on its own as it was getting all its nutrients directly from Itzmara's heart. It was around 5 centimeters (2 inches) long, had a rudimentary head, and limbs but no brain or heart.

First described by German anatomist Johann Friedrich Meckel in the late 18th century, there have been fewer than 200 medical cases reported of fetus in fetu, so when it does occur it makes headlines around the world, like the baby girl from Cote d'Ivoire in 2017 who had surgery to remove her parasitic twin, as its lower limbs were growing out of her back, and a 17-year-old girl earlier this year whose growing lump in her abdomen revealed a 30-centimeter (12-inch) parasitic twin, which was successfully removed.