Parasitic Twin Facts: Are Vestigial Parasitic Twins Alive and Conscious?

    Published 30 April 2020
    Fact Checked
    Reviewed by Eugenia Tikhonovich, MD, Obstetrician-Gynecologist, Medical Consultant
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    Unlike symmetrically conjoined or vanishing twins, parasitic twins appear in the form of nonconscious tissue or extra limbs on autositic twins. Learn more about this fascinating phenomenon with Flo. 

    What is a parasitic twin?

    A vestigial parasitic twin is actually a type of conjoined twin, which isn’t fully formed and relies entirely on the autositic or complete twin’s body. Note that there are two main versions of conjoined twins:

    • Equal or symmetrical  

    Two equally well-developed fetuses

    • Unequal or asymmetrical 

    Either a partially developed twin attaches to a fully developed twin, or one small body part gets duplicated 

    When one conjoined twin stops growing during pregnancy, they become rudimentary to the dominant or autositic twin, resulting in a parasitic twin. The parasitic twin is either nonfunctional, incompletely formed, or totally dependent on their sibling. Consequently, vestigial parasitic twins are physically composed of a single, properly functioning fetus who carries extra organs or appendages.

    Parasitic conjoined twins don’t typically share their organs and their fetal parts are grossly recognizable. A pedicle of soft tissues containing large blood vessels connects the parasitic twin to the other twin. Often times, a parasitic twin will be named based on the site of attachment:

    • Cephalopagus

    At the back of the head

    • Epigastric 

    At or above the upper abdomen

    • Omphalopagus 

    At the abdomen

    • Craniopagus parasiticus 

    At the skull or cranium 

    • Ischiopagus

    At the pelvis

    • Parapagus 

    At the abdomen and pelvis

    • Pyopagus 

    At the lower spine

    • Rachipagus 

    At the back

    • Thoracopagus 

    At the upper chest

    • Pygomelia

    Extra limbs attach at the buttocks and may resemble a false tail

    Without treatment, the dominant twin is vulnerable to serious medical complications from supporting the vestigial twin.  

    How are conjoined twins different from parasitic twins?

    Two newborns physically connected to one another at birth will be referred to as conjoined twins. They’re the product of a partial separation of the early embryo into two fetuses. Although they both grow in utero, they remain attached (usually at the abdomen, chest, or pelvis), and occasionally share one or more internal organs.

    How are vanishing twins different from parasitic twins?

    Vanishing twins or vanishing twin syndrome describes a multifetal pregnancy with the subsequent disappearance of one of the fetuses. The condition has been more frequently diagnosed since the advent of ultrasound technology in early gestation. After the twin vanishes, a compressed or mummified fetus forms or the surviving twin completely absorbs them. In some instances, it produces a placental abnormality such as a cyst.

    What leads to parasitic twins?

    Two dominant theories exist to explain the creation of parasitic twins: the fission theory and the fusion theory. The former proposes a partial separation of a single embryo (at the end of the second week following conception). On the other hand, the latter points to the combination of two initially distinct parts in early development.

    Additionally, some experts suggest that incomplete twinning is responsible for parasitic twins. A single egg fails to fully separate or split (between days 13 and 15 after conception), ultimately producing conjoined twins.

    Lastly, ischemic atrophy theory cites vascular problems in the uterus as the underlying reason for ischemic accidents and the development of a parasitic twin.

    At this point, it’s still unknown what triggers any of the above scenarios.

    How is a vestigial twin diagnosed?

    Your health care provider may diagnose parasitic twins with a standard ultrasound by the end of the first trimester. Echocardiograms and more comprehensive ultrasound tests could be used around the halfway-mark of your pregnancy. They help determine the degree of the parasitic twin’s connection and their organ function.

    The discovery of conjoined twins often warrants a magnetic resonance imaging (MRI) exam to investigate the site of attachment and whether they share any organs.

    However, even with the aid of imaging technology, a parasitic twin cannot be found or confirmed with absolute certainty. It’s possible to miss a small parasitic twin, which might simply appear as a tissue mass on a single fetus. And the rarity of the parasitic twin phenomenon makes medical professionals even more likely to overlook it.

    That’s why it’s important to begin monitoring your fetus early on to spot potential complications and secure an accurate diagnosis.

    Are parasitic twins alive and conscious?

    Naturally, you may be wondering, “Are parasitic twins conscious?” The answer is no, parasitic twins are not conscious, and despite remaining attached to their dominant sibling, they cannot survive independently.

    Parasitic twin removal

    Surgical removal of a parasitic twin is generally performed to save the life of the autositic twin and preserve their health. It relieves the physical burden of the parasitic twin on their healthier sibling and is usually done after delivery.

    Each case of conjoined parasitic twins features unique circumstances. Surgical procedures for parasitic twin removal vary depending on the extent and site of connection. Several imaging tests are conducted in an effort to map the surgery carefully before beginning. Surgeons must not only separate soft tissues, but bony as well as vascular connections (i.e., those containing blood vessels). 

    Issues arising from parasitic twin removal may include infection, hernia, and improperly healed wounds. In some instances, follow-up surgery is required.

    Afterwards, the dominant twin is tested for breathing and heart problems caused by the strain of supporting the vestigial parasitic twin. Otherwise, the autositic twin tends to possess an excellent chance of survival.


    Parasitic twins are an asymmetrical version of conjoined twins in which a partially formed fetus is attached to a normal, functioning fetus. Although organ sharing between them is very rare, the parasitic twin sometimes displays a fully functioning heart or brain. In other instances, the autositic twin looks like a single fetus with unrecognizable protrusions or extra limbs. 

    If the parasitic twin condition is left untreated, the dominant sibling is susceptible to major medical complications. 

    Since the rate of parasitic twins is extremely low ‒ occurring in less than one out of every 1 million births ‒ research is scarce. However, with the help of improved surgical and imaging techniques, experts continue striving to unravel the many mysteries surrounding parasitic twins.

    History of updates

    Current version (30 April 2020)

    Reviewed by Eugenia Tikhonovich, MD, Obstetrician-Gynecologist, Medical Consultant

    Published (30 April 2020)

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