Parasitic Twin

A parasitic twin is a very rare condition that happens when one twin doesn’t fully develop. It’s a type of conjoined twin pregnancy. The parasitic twin stays attached to the other fetus, which continues to grow until birth. It affects fewer than 1 in 1 million births worldwide.

The parasitic twin has no chance of survival. The other twin — the dominant (autositic) twin — fully develops. But the dominant twin has an increased risk of medical complications.

The dominant twin is born with extra limbs, organs or tissue from the parasitic twin. They usually need surgery shortly after birth to remove the tissue.

Healthcare providers may also call parasitic twins:

• Asymmetrical conjoined twins
• Heteropagus twins
• Vestigial twins

It’s important to note that a parasitic twin isn’t the same as vanishing twin syndrome. In this syndrome, the undeveloped twin is completely reabsorbed by the mother’s body or the placenta.

What does a parasitic twin look like?

A parasitic twin consists of underdeveloped body parts that are attached to the dominant twin. Most often, these are arms and legs, but it can vary. Sometimes it can look like a mass or lump that you can’t identify. The tissue can contain underdeveloped organs and bones.

The parasitic twin tissue can be attached anywhere on the dominant twin. Healthcare providers classify the parasitic twin based on where the tissue attaches:

• Cephalopagus: From the top of the head down to the belly button
• Craniopagus: At the skull
• Epigastric: The upper part of the belly
• Ischiopagus: At the pelvis
• Omphalopagus: At the belly (most common)
• Parapagus: At the torso
• Pygomelia: At the butt
• Pyopagus: At the lower back or spine
• Rachipagus: At the middle back
• Thoracopagus: At the chest

Two other types of parasitic twins include:

• Fetus in fetu: The malformed fetus is inside the body of the dominant twin rather than attached to the outside. It’s often mistaken for a tumor (teratoma). You might not know you have it until later in childhood or even adulthood.
• Acardiac twin (TRAP sequence): One twin doesn’t have a functional heart. But this twin gets blood supply from the other twin (the “pump twin”) and can continue to increase in size.

Parasitic twin causes

Researchers don’t know for sure what causes a parasitic twin pregnancy. But the main theory is that part of the embryo doesn’t completely divide very early in an identical twin pregnancy. This results in the dominance of one embryo's development over the other.

A blood flow issue causes parts of the parasitic twin to waste away. The parts of the parasitic twin that remain rely on the blood supply from the dominant twin where they’re attached. The parasitic twin usually doesn’t have a heart and very rarely has brain tissue.

There’s nothing you can do to prevent this.

Complications of this condition

The dominant twin is at increased risk for complications. Congenital heart disease is the most common. They may have other complications depending on where the parasitic twin is attached, like:

• Omphalocele: Your baby’s abdominal organs stick out through an opening at their belly button. A see-through membrane covers these organs.
• Myelomeningocele: Your baby’s backbone and spinal canal don’t close before birth. It’s a type of spina bifida.
• Tethered spinal cord: Your baby’s spinal cord abnormally attaches to the wall of their spinal canal.

These conditions are treatable. Your baby’s healthcare team will explain the kind of treatments they’ll need.

How doctors diagnose this condition

Healthcare providers typically diagnose parasitic twins during pregnancy ultrasounds. Your provider may recommend other tests to see how the parasitic twin is attached and to look for any health issues in the dominant twin. These tests may include:

• MRI
• CT scan
• Fetal echocardiogram

Your healthcare team can use these tests to plan for separation surgery after birth and any other care your baby may need. They may also recommend a scheduled C-section.

In some cases, your provider may not be able to see the parasitic twin in an ultrasound. It may be too small or appear as a lump or mass of tissue. They’ll diagnose it after you give birth to the dominant twin.

What should I ask my healthcare provider?

You may have a million questions for your healthcare provider. Consider using these to start with:

• How high is the risk to the dominant twin?
• How often will I need monitoring?
• What are the risks of surgery?
• What are the survival rates for the dominant twin?

How is it treated?

Your baby will need surgery to remove the parasitic twin. Leaving the tissue attached can cause breathing, heart and growth problems. The surgery usually happens within a few days after birth.

Each case is different. Your healthcare team will walk you through the steps of the surgery and possible complications. They’ll let you know if your baby needs any additional care.

What can I expect if I have a parasitic twin pregnancy?

A parasitic twin pregnancy is very rare, and each case is unique. So, it’s difficult for providers to make predictions about the dominant twin’s outlook from the start. You can expect a lot of appointments and tests. Know that your healthcare team will keep you informed every step of the way.

Allow yourself time to process this diagnosis. Reach out to your support network to help process what you’re feeling. It’s a good idea to reach out to a therapist or grief counselor for support, too.

It’s OK to feel grief and hope at the same time.

Prognosis

Each situation is different. But, overall, the dominant twin generally has a good outlook (prognosis).

The outlook mainly depends on:

• Where and how the parasitic twin is attached
• Whether the dominant twin has other health issues, like congenital heart disease

Surgery complications, like sepsis and incisional hernias, can also affect your baby’s outlook. In one study that looked at 120 cases of parasitic twins, 12 dominant twins died. That means 9 out of every 10 dominant twins lived.

Long-term follow-up is often necessary to monitor for cosmetic or reconstructive needs (plastic surgery). Your child may also need mental health support as they learn about their birth history.