A baby girl born recently in Colombia is among 100-odd known cases of “fetus-in-fetu”—a very rare phenomenon in which a malformed fetus is found inside the body of its twin.
As Donald G. McNeil Jr. reports for the New York Times, the anomalous growth was detected in-utero, and doctors initially believed that the 35-week-old fetus had developed a liver cyst. Using color Doppler and 3D/4D ultrasound imaging, however, high-risk pregnancy specialist Miguel Parra-Saavedra was able to determine that the “fluid-filled space” actually contained the body of a tiny infant, attached by an umbilical cord to its sister’s intestine.
The baby, whose name is Itzmara, was delivered via C-section at 37 weeks, three weeks shy of full term because doctors feared that the fetus inside her, which was still growing, would crush her internal organs. The twin was then removed by laparoscopic surgery. It was two inches long, with half-formed head and limbs, but did not have a heart or brain.
Fetus-in-fetu only occurs in about one in every 500,000 births, but it has been documented around the world for many years, with one known case dating back to the early 19th century. The condition only happens among identical twins, which split from a single fertilized egg and share a placenta.
Embryos start off as flat discs, which, at around the fourth week of gestation, fold in various directions to form “shapes that can eventually form body structures and organs,” explains Rachel Feltman of Popular Science. Because identical twin embryos grow in close proximity, very rarely one can get folded into the other during this biological process; according to New Scientist’s Alice Klein, this may be more likely to happen if the absorbed twin has an existing defect.
Often this smaller twin—known as a heteropagus or “parasitic” twin because it takes nutrients from its sibling—grows in the abdomen, which is rich in blood supply. But parasitic twins have been found in other parts of the body, including the cranium. Nor are parasitic twins always internal; McNeil Jr. reports that they can be conjoined to their developed sibling, or grow partially inside and partially outside the other twin’s body.
Fetus-in-fetu is different from the phenomenon known as teratoma, a type of embryonal tumor that can contain hair, bones, teeth and even eyes. There is some debate as to how the conditions should be distinguished and diagnosed, but Corinne DeRuiter of the Embryo Project Encyclopedia explains that with fetus-in-fetu, “there must be evidence of body plan organization, including vertebrae, limb buds, and organ tissues.”
There have been cases where parasitic twins have gone undetected for decades. In 2015, for example, a 45-year-old woman underwent surgery to have a mass removed from her left ovary; the mass was found to have a face, an eye, a tooth and a long, black hair. But thanks to advanced ultrasound techniques, medical professionals are now often able to spot instances of fetus-in-fetu in-utero and remove the parasitic twin shortly after birth—as was the case with baby Itzmara. Parra-Saavedra, who oversaw her delivery, tells the Times’ McNeil Jr. that she has “a little scar on her abdomen,” but is otherwise doing well.