Doctors in China’s Shanghai removed an “unborn twin” from the brain of a one-year-old girl, according to a study published in Neurology, a peer-reviewed journal. It revealed that the child was brought in after she presented problems with motor functions and had an enlarged head.

While such cases are rare, they have been found earlier as well. For instance, in 2015, a 26-year-old student in the United States was diagnosed with teratoma, a growth which had hair and bone. Doctors described it as an embryologic twin “extremely deep in the brain”, BBC reported. In November last year, eight embryos were removed from the stomach of a 21-day-old infant in Ranchi, Jharkhand.

Talking about the same, Dr Parimala Devi, Senior Consultant, Obstetrics and Gynaecology, Fortis Hospital, Bannerghatta Road explained that a parasitic twin is a rare condition that occurs when one twin develops incompletely and becomes dependent on the other twin for survival. “In this case, the parasitic twin was found in the brain of a one-year-old. It’s believed that the parasitic twin began developing in the womb and grew inside the surviving twin’s body,” she said, talking about the recent case.

Dr Devi added that parasitic twins are an extremely rare form of conjoined twins, also known as asymmetric conjoined twins. “Unlike conjoined twins, parasitic twins are not fully formed and depend on the body of the other twin for survival.”

But, what causes this? “The exact cause of parasitic twins is not fully understood, but it is believed to occur when an embryo begins to develop into twins, but the process is not completed,” the expert said, adding that this can result in one twin growing within the body of the other twin.

Agreeing, Dr Ruby Sehra, Senior Consultant, Gynaecology at Sri Balaji Action Medical Institute said, “Such conditions form during an identical-twin pregnancy when one of the fetuses is absorbed by the other early in pregnancy. It happens as part of the fetal development process when there are cavities that close during development and one of the embryos enters a space.”

They typically occur early in pregnancy, between weeks 4 and 8 when the embryo is dividing into multiple cells and forming the basic structure of the body. “A parasitic twin can be difficult to diagnose early in pregnancy because it is a rare condition and may not present any obvious symptoms. However, prenatal ultrasound imaging can be used to detect the presence of a parasitic twin,” Dr Devi said. Other tests could include magnetic resonance imaging (MRI) or a computerised tomography scan (CT scan), Dr Sehra added.

However, in some cases, the condition may not be diagnosed until after birth, as in the China case. Dr Surabhi Siddhartha, Consultant Obstetrician and Gynecologist, Motherhood Hospital, Kharghar, said, “Even with imaging studies, determining the presence of a parasitic twin might be challenging. A much tiny parasitic sibling can easily be overlooked. It might even look like a mass on a single infant. If a parasitic twin is found, foetal echocardiography on the host twin can be conducted. This is done because caring for a parasitic twin can place significant pressure on the heart. Imaging tests may not be conducted at all if prenatal care is inadequate. Conjoined twins or parasitic twins may not be detected correctly until they are born.”