A parasitic twin is an identical twin that has stopped developing during gestation, but is physically attached to the fully developing twin. The fully developed twin is also known as the dominant or autosite twin.

The parasitic twin never completes development. Rarely, if ever, do parasitic twins have a complete brain or functional heart. They usually don’t share organs with the dominant twin. They can’t survive on their own.

In some cases, the dominant twin looks like a normally developed baby with extra limbs or unrecognizable protrusions. Without treatment, this twin may develop severe health problems from the efforts of supporting the parasitic twin.

Parasitic twins are also known by other names, including:

  • abnormal twinning
  • asymmetric conjoined twins
  • fetus in fetu
  • vestigial twins

Parasitic twins occur in fewer than 1 in 1 million births. Because it’s so rare, there’s not a lot of documentation for researchers to go on. Many questions remain, but improved imaging and surgical techniques will help doctors gain insights into the mysteries of parasitic twins.

Vs. conjoined twins

While parasitic twins are a type of conjoined twin, conjoined twins don’t always involve a parasitic twin. Conjoined twins, also called symmetrical twins, both have developed brains.

They share some body parts, but they’re two functioning individuals. Whether they can be surgically separated or not, both twins can survive.

Vs. vanishing twins

Parasitic twins differ from vanishing twins. With vanishing twins, early ultrasound or fetal heartbeat confirms the presence of two fetuses. But in later tests, only one fetus remains.

The vanishing twin is absorbed by the remaining twin, the placenta, or the mother’s body. It can also be compressed by the healthy twin.

Vanishing twins are known as twin embolization syndrome or vanishing twin syndrome. They can be identical or fraternal.

Researchers have a long way to go to fully understand how this happens. That’s because there are so few well-documented cases and each one is unique.

Identical twins come from a single fertilized egg that splits in two shortly after fertilization. The result is two separate fetuses. If the egg fails to fully separate, the result is conjoined twins.

Sometimes, one of the fetuses is partially absorbed by the other in early pregnancy. The partially absorbed fetus stops developing and becomes parasitic. The other twin continues to develop normally and becomes dominant.

While there are many theories, such as gross deformity or vascular compromise in utero, what triggers this chain of events is unclear.

There are no obvious symptoms of a twin pregnancy that involves a parasitic twin.

It’s possible to identify a parasitic twin during your pregnancy. Much depends on the positioning and extent of the developmental abnormality as can be seen on imaging tests such as:

Even with imaging tests, it can be difficult to say with certainty that there’s a parasitic twin. A much smaller parasitic twin can be missed entirely. It might even appear to be a mass on a single baby. And most doctors have never seen one firsthand.

If a parasitic twin is detected, fetal echocardiography can be performed on the autositic twin. This is done because supporting a parasitic twin can put enormous strain on the heart.

When prenatal care is lacking, imaging tests may not be performed at all. Conjoined twins or parasitic twins may not be properly identified until they’re born.

The majority of cases of parasitic twins involve externally attached, motionless limbs. Dipygus is a term that describes a twin that appears to have extra limbs, extra sexual organs, or double buttocks.

Fetus in fetu is a parasitic twin that’s completely enclosed inside the body of the healthy twin. This can go undiscovered at birth, only becoming obvious later. Fetus in fetu may initially be thought to be a tumor. It’s a rare condition with fewer than 200 cases having been reported worldwide.

Types of parasitic twin are also named for the location of attachment:

  • cephalopagus: back of the head
  • craniopagus parasiticus: cranium
  • epigastric: upper abdomen, above the stomach
  • ischiopagus: pelvis
  • omphalopagus: abdomen
  • parapagus: trunk
  • pygomelia: limbs attached to buttocks; might appear as a pseudo tail
  • pyopagus: lower spine
  • rachipagus: back
  • thoracopagus: upper chest

A parasitic twin is partially absorbed by the autositic twin in the early stages of development in the womb. It won’t be fully absorbed by the healthy twin after it’s born.

Currently, there’s no in utero treatment due to the complexities of diagnosis and the potential range of abnormalities. But there’s value to diagnosis prior to birth.

As detailed in a 2004 case report, a parasitic twin was identified with ultrasound in the 28th week of pregnancy. The dominant twin appeared normal with two abnormally formed parasitic legs at the lower spine. The dominant twin’s legs moved freely. No movement was detected in the parasitic legs.

Doctors were able to counsel the parents, manage the pregnancy, and plan for a cesarean delivery. Soon after birth, the parasitic legs were surgically removed without complication.

If not identified during pregnancy, a parasitic twin is usually evident at birth.

The goal of treatment is to save the life and preserve the health of the autositic twin. To ease the burden on the healthy twin, surgical removal of the parasitic twin is necessary.

Each case has a unique set of circumstances. Surgery techniques depend on the site and extent of the connection. Experienced neurosurgeons must use imaging tests to carefully map out the surgery.

Bony and soft tissue connections, as well as any vascular connections, must be separated. There are usually no shared organs.

Potential complications from the surgery vary, depending on the specific details. These may include wound complications, hernia, and infection. In some cases, follow-up surgeries may be needed.

The autositic twin may be tested for heart defects that could arise from supporting the parasitic twin. Aside from cardiorespiratory problems, the chances for survival of the autositic twin are excellent.

Fetus in fetu may not be diagnosed until it becomes obvious or causes an issue. Most of the time, it’s discovered as an abdominal mass in the first year of life, but it’s also been reported in adults.

Fetus in fetu can lead to failure to thrive, infection, and problems with organ function. The treatment is surgical removal.

Parasitic twins are big news, so they may seem more common than they are. They’re actually such a rare occurrence that most doctors have never come across a case.

In 2017, a baby named Dominique made headlines when she traveled from West Africa to Chicago for surgery. There, a team of neurosurgeons spent six hours removing her parasitic twin. The 10-month-old had an extra pelvis, legs, feet, and toes protruding from her neck and back.

As reported by The Washington Post, the strain of supporting the extra limbs would have shortened her life. After the surgery, Dominique was expected to have a normal life.

That same year, an unnamed baby boy from India underwent successful surgery to remove his parasitic twin. During the procedure, a team of 20 doctors removed two extra legs and an extra penis.

CNN reported that the boy would need follow-up care, including corrective surgery. He was also expected to have a normal childhood.