A parasitic twin is a type of conjoined twin where one fetus stops developing but remains attached to its twin. The other twin continues to develop, but is usually born with the limbs, organs or other tissue structures from its parasitic twin still attached. It’s a very rare condition.
A parasitic twin is a rare condition that happens when a conjoined twin stops developing. The parasitic twin never fully develops but stays attached to its twin, who continues to develop until birth. A parasitic twin happens early in embryonic development, so its organs or limbs can be nearly unrecognizable. This results in the healthy, developed twin (the dominant or autositic twin) being born with extra limbs, organs or tissue from its parasitic twin. The dominant twin is at high risk for medical complications and requires medical treatment after birth.
A parasitic twin is also known as:
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The type of parasitic twin is named for where it attaches or joins to the dominant twin.
A parasitic twin is a type of conjoined twin. It accounts for about 10% of all conjoined twins. Conjoined twins are two fully developed fetuses that are connected at birth. Conjoined twins share one or more organs and are usually attached at their back, chest or torso. A parasitic twin is one fully developed fetus and one undeveloped fetus. The undeveloped fetus is nonfunctional and doesn’t survive the pregnancy.
A parasitic twin is very rare. It affects less than 1 in 1 million births worldwide.
There are no symptoms of parasitic twins.
No one is entirely sure, but researchers point to two main theories — the fission theory and the fusion theory.
The fission theory describes an incomplete separation of one fertilized egg. When a fertilized egg fully separates, the result is identical twins. If a fertilized egg partially splits or doesn’t fully separate, it results in conjoined twins. A parasitic twin happens when one of the fetuses in a conjoined twin stops developing.
The fusion theory suggests two separate fertilized eggs grow or fuse together during development. A parasitic twin occurs when one of these two fertilized eggs stops developing but remains attached to its twin.
Another theory suggests that it occurs due to vascular problems within your uterus during fetal development.
Your healthcare provider may diagnose parasitic twins in a prenatal ultrasound. Further testing may be needed to determine how the parasitic twin connects to the dominant twin and what organs are involved. Other imaging tests could include magnetic resonance imaging (MRI) or a computerized tomography scan (CT scan).
If a parasitic twin is suspected, an echocardiogram may be necessary to determine if the parasitic twin is putting strain on the autositic (dominant) twin’s heart.
In other cases, a parasitic can’t be seen until the dominant twin is born. The parasitic twin may be too small or appear as a lump or mass of tissue.
If left untreated, the dominant twin could be susceptible to diseases and complications from supporting its parasitic twin. Once the dominant twin is born, healthcare providers evaluate them to ensure their heart, lungs and other organs are working properly. Each situation is different and depends on how the parasitic twin has attached and the extent of the attachment.
Surgical removal of the parasitic twin from the dominant twin is necessary. A surgeon removes the bones, organs and tissues from the surviving twin. This procedure has several risks like infection, hernia or other surgical complications.
Parasitic twins are unpreventable. It occurs during fetal development.
A parasitic twin isn’t alive and dies in the womb while attached to the dominant twin. The surviving, dominant twin may need comprehensive medical treatment to remove tissue, organs or limbs from its parasitic twin. The surviving twin may have respiratory or cardiovascular problems that require monitoring. The instances of parasitic twins are rare, but the dominant twin usually survives.
A vanishing twin is when one twin gets absorbed during the pregnancy, ending its development. It’s absorbed either by the placenta, the other twin or your uterus. With vanishing twin syndrome, an ultrasound shows two fetuses in early pregnancy. Later on, only one fetus remains. With a parasitic twin, the twin doesn’t absorb, and parts of its body remain attached to the dominant twin. Vanishing twins aren’t conjoined twins. Parasitic twins are a type of conjoined twin.
The parasitic twin doesn’t survive the pregnancy. The dominant twin is fully developed and typically survives.
A note from Cleveland Clinic
A parasitic twin is a rare type of conjoined twin. It occurs when a partially formed fetus attaches to another fetus (its twin) very early in development. One fetus grows normally, while the partially formed fetus stops developing but remains connected to its twin. The healthy, surviving twin may be born with extra limbs or have a mass of tissue where its sibling was. A parasitic twin must be removed from its twin to prevent major medical problems. Not much research exists about this condition because it’s so rare. Healthcare experts are learning more about parasitic twins, but it’s expected to remain a rare anomaly.
Last reviewed by a Cleveland Clinic medical professional on 07/19/2022.
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