What is a “Parasitic Twin” & Why Don't We Say This Anymore?

What is a “Parasitic Twin” & Why Don't We Say This Anymore?

Your pregnancy week by week

The term “parasitic twin” refers to a rare type of conjoined twin where one twin in the pair stops developing inside the womb.
Conjoined twins are twins that are physically connected to one another. Each twin has their own brain and, although they share body parts, they are usually able to function somewhat independently. They are a rare phenomenon—happening once in every 50,000 to 60,000 births.

Sometimes one of the conjoined twins is unable to fully form in the womb, resulting in what has become known as a “parasitic twin”. This is even more rare, occurring in about 10% of cases of conjoined twins.

What is a parasitic twin?

The term “parasitic twin” has been used for a while to describe what we prefer to call an underdeveloped conjoined twin. (Yep, we’ve had it with the unnecessarily negative language around fertility and motherhood and are—finally!—mixing it up. Head here for more on our Renaming Revolution.)

An underdeveloped twin can be detected on a CT, MRI, or ultrasound scan. Because the underdeveloped twin is so reliant on the dominant twin, it can put a lot of strain on the dominant twin—particularly when it comes to heart function. (Basically, one little heart has to pump for both.)

There are various types of underdeveloped conjoined twins:

  • Vestigial twins. These are the most common and will often appear as a baby with additional limbs or organs.
  • Dipygus twins. They also have extra legs, and sometimes additional hands, feet, and sexual organs.
  • Craniopagus. This is when an additional head is attached to the head of the dominant twin. (The head may have a body, but the organs inside that body will not function in a way that can sustain life.)
  • Epigastric twins. In this case, additional body parts may be attached to the dominant twin’s abdomen.
  • Fetus in fetu (fetus inside a fetus). This type can be far more difficult to detect and may first be diagnosed as a tumor. Essentially, one twin is inside the other twin’s body.

What is the cause of parasitic twins?

The topic is still not that well understood—one of the reasons being that there are so few cases out there and another that each case is unique.

To get our heads round what might cause underdeveloped conjoined twins to happen, let’s quickly go through the basics of how twins are formed.

Essentially, there are two main kinds of twins: monozygotic (identical) and dizygotic (non-identical or fraternal) twins.

Dizygotic twins are formed when two eggs are released and fertilized by two different sperm cells, making them as genetically similar as a sibling pair born at different times.

Monozygotic twins are formed when one egg is fertilized and then splits—resulting in two babies who are genetically identical.

In some cases, the egg doesn’t separate completely—and so we get conjoined twins. Sometimes these twins are symmetrical, in that, while they are attached, they’re both able to develop the organs they need to survive independently.

At other times, one of the fetuses is partially absorbed by the other. While one twin develops and grows, the other one becomes fully reliant on their conjoined sibling for their survival. This is one of the more popular theories for how an underdeveloped conjoined twin comes about.

Is a parasitic twin alive?

This is a difficult question to answer. The brain and heart of an underdeveloped conjoined twin don’t develop to the point where the twin can exist independently. So, they are alive in a sense, but they can’t survive without the other twin.

Is a parasitic twin a parasite?

NO! We are so against the use of this kind of language because:

  1. It distracts from the fact that this is actually a form of pregnancy loss, in that one of your babies will not be brought to term, and
  2. Your surviving baby will be put under a lot of strain to support their underdeveloped sibling. This is already difficult to deal with, and callous language denies the existence of the pain, complications, and grief that can be involved here.

A vanishing twin (or what we prefer to call a “miscarried multiple” for the same reasons we’re steering away from the language of “parasitic twin”) is a twin that doesn’t survive in the womb. So an underdeveloped conjoined twin is often seen to be somewhere between a conjoined twin and a miscarried multiple.

Either way, it means loss—and if this is what you’re going through at the moment, make sure you get the support you need. Counseling, relying on friends and family, and upping the self-care are all important right now.

It’s really time we changed up the negative language around motherhood and fertility. Join the #renamingrevolution and let’s start having the conversation—with words that do less harm.

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