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

The term “parasitic twin” has been used for a while to describe what we prefer to call an underdeveloped conjoined twin.

Don't worry, we’ve also 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.

In case you're not sure, conjoined twins are twins that are physically connected to one another.

Each twin has their own brain, and although they share body parts, they're usually able to function somewhat independently.

And they're a rare phenomenon—happening once in every 50,000 to 60,000 births.

But sometimes one of the conjoined twins is unable to fully form in the womb, resulting in what has become known as a “parasitic twin”.

Here's what you need to know.

In this article: 📝

What is a parasitic twin?

A parasitic twin (or asymmetrical twin) refers to a rare type of conjoined twin where one twin in the pair stops developing inside the womb.

It's incredibly rare, happening in less than 1 in 1 million births worldwide, and 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.

What are the symptoms of a parasitic twin?

There's no signs or symptoms to clue you in on whether you have an asymmetrical twin.

But, conjoined twins can be diagnosed early in pregnancy using ultrasound.

From here, your healthcare provider will utilize more detailed ultrasounds to determine the extent of the twins' connection and the functioning of their organs.

What is an example of a parasitic twin?

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 causes 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.

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.

Can you give birth to conjoined twins naturally?

First off, each situation is unique and not just because you're experiencing a rare circumstance.

Your twins' health issues, where they are joined along with which organs and structure they share, will all impact your pregnancy and delivery.

If you are a mama carrying conjoined twins, expect to be referred to a specialist and be closely monitored.

Depending upon your babies, this information could be different, but a lot have a planned c-section often 3-4 weeks before the estimated due date.

Once your twins are born, they will be evaluated, and decisions regarding their care will be made.

No doubt, your postpartum recovery will look and feel very different from how you imagined.

Counseling, relying on friends and family, and upping your self-care are all important right now.

And if you ever need a space to connect, the Peanut community is always here.

We've got you. ❤️