What Are Parasitic Twins?

Medically Reviewed by Poonam Sachdev on January 18, 2023
4 min read

A parasitic twin is an extremely rare type of conjoined twin. While research is limited, experts suggest that this condition doesn’t cause severe complications. But it requires surgery — often in the first few days after the baby is born. Here’s what you need to know.

A parasitic twin is a rare condition where a baby is born with an underdeveloped twin attached to its body. Also known as “vestigial twins,” this condition is closely related to conjoined twins — babies that are connected at birth and share organs. The main difference is that conjoined twins present two developed babies, while parasitic twins require one of them to be severely underdeveloped and nonfunctional.

The developed, functional twin is medically known as the autositic or dominant twin, while the underdeveloped one is simply referred to as the parasitic twin. The dominant twin is usually healthy in most aspects but may have extra tissue, organs, or limbs from the parasitic conjoined twin.

The parasitic twin may be attached to the dominant one in one of several places. They commonly join at the head, torso, abdomen, chest, pelvis, buttocks, or back. It’s important to remember that the parasitic twin isn’t alive — they almost always die during birth or even while in the womb.

Parasitic twins are extremely rare — experts estimate they happen in fewer than 1 in 1 million babies. While the condition may sound severe, modern medical advancements present a positive outlook for the developed baby. In most cases, doctors must surgically remove the parasitic twin to avoid complications.

Two other conditions can be confused with parasitic twins: fetus-in-fetu and mature teratoma. The first refers to a skin-covered, fetuslike abnormal mass that’s mostly inside the newborn. Mature teratomas are, instead, a type of cancer that may become malignant.

Because it’s an extremely rare condition, researchers don’t know the exact mechanisms that cause it to appear. But there are two leading theories regarding parasitic twin causes, both relating to the early development of the fetus.

Fission theory. The first theory suggests that the parasitic twin appears during the early stages of pregnancy, usually at the end of the first two weeks. At this time, the fertilized egg sometimes splits into two separate parts, resulting in identical twins. But experts point out that if the egg doesn’t entirely separate and one of the parts stops developing, it results in a parasitic twin.

Fusion theory. The second theory proposes the inverse mechanism, where the egg successfully separates, but both parts come in contact with each other at a later stage. If this happens, their cells may start to interact, resulting in conjoined twins. According to this theory, a parasitic twin appears when one of the two parts stops developing.

Researchers have also started studying the impact of the Sonic Hedgehog protein (SHH) on the development of parasitic twins. The SHH protein family is a fundamental compound involved in the development and maintenance of cells. 

Abnormalities in SHH levels can lead to several conditions in a baby, like fused eyes, legs, and feet. Experts suggest that high levels of SHH proteins can lead to parasitic twinning. But more research is needed to fully understand the causes of parasitic twins.

Unfortunately, no symptoms can suggest that you’ll give birth to a parasitic twin. Instead, you have to rely on medical tests during pregnancy. These primarily consist of imaging tests, like prenatal ultrasounds, magnetic resonance images (MRIs), and computerized tomography (CT) scans.

During these tests, your doctor will be able to tell if any abnormalities in your baby are similar to a parasitic twin. In the case of a positive diagnosis, you may be asked for an echocardiogram and other tests to ensure that the parasitic twin isn’t causing trouble in the dominant one.

But it’s essential to know that, sometimes, it’s impossible to see and distinguish a parasitic twin while the baby is still in the womb. This can lead to doctors diagnosing the condition immediately after you give birth after they detect the lump of tissue.

The only possible treatment for a parasitic twin is surgical excision (removal). While this may sound too harsh for a newborn baby, leaving a parasitic twin untreated can cause many complications. But doctors may not operate as soon as the baby is born due to late diagnosis or for precaution.

During the surgery, the doctor removes all of the parts belonging to the parasitic twin from the dominant one. This may include tissues, bones, and even organs. In some cases where the parasitic twin causes deformities, your baby may also need surgical reconstruction of specific body parts.

Thanks to modern advancements in surgical techniques, most babies don’t have any immediate postoperative complications. Yet, some cases may need more intensive post-operative monitoring, including precautions like artificial ventilation.

Like any surgery, the removal of a parasitic twin can cause several complications. However, it’s important to remember that most babies with parasitic twins have a positive medical outlook. Here are some of the most common complications that arise from surgery procedures in newborns:

  • Infections
  • Hernias
  • Abnormalities in blood flow
  • Multiple organ dysfunction syndrome
  • Respiratory failure

If you have any doubts concerning parasitic twins, check with a doctor. A trained professional will be able to answer your questions so you can remain calm until your baby is born. But keep in mind that parasitic twins are extremely rare — so you may have to consult a pregnancy specialist.