What Are Conjoined Twins?

Written by Kathryn Whitbourne
Medically Reviewed by Traci C. Johnson, MD on May 02, 2025
8 min read

Conjoined twins are twins who are physically attached to one another. This condition can cause complications during pregnancy, delivery, and after the babies are born.

Some conjoined twins can be separated. Children’s Hospital of Philadelphia (CHOP) has successfully separated 32 pairs of conjoined twins since 1957.

"To determine if conjoined twins can be safely separated, we study their shared structures using detailed imaging," says Holly L. Hedrick, MD, an attending pediatric and fetal surgeon in the Division of Pediatric General, Thoracic and Fetal Surgery at Children's Hospital of Philadelphia (CHOP). She has worked with two teams at CHOP to separate conjoined twins. "This begins before birth with specialized tests, like a fetal echocardiography to map the heart, along with ultrasound and MRI to assess shared organs."

The best outcome is when conjoined twins can be safely separated, but not all conjoined twins can be separated. Sometimes, where they're joined on their bodies makes it impossible, and they stay connected for life. Most conjoined twins need a lot of medical care to help manage their long-term health.

"After birth," Hedrick says, "we repeat imaging with tools like echocardiograms, MRIs, and other scans as needed, depending on where the twins are conjoined." The most common location twins are joined, she says, is at the chest and abdomen.

Conjoined twins were originally called Siamese twins because of a famous pair from Thailand (formerly Siam) named Chang and Eng Bunker. They were performers with a traveling exhibition in Europe and America in the 1800s. They married sisters, had 22 children between them, and by all accounts had successful lives.

Scientists aren't sure how it happens. But there are a few theories:

  • Fission: This idea suggests that an early embryo contains a sphere of identical cells. The sphere splits into two, but never completely separates, developing into conjoined twins.
  • Fusion: This theory says that conjoined twins are monozygotes, meaning they begin as one fertilized egg that splits into two embryos — identical twins. But early on, the twin embryos join at some random point and become conjoined twins.

Conjoined twins are very rare, and happen in about one in every 50,000 to 60,000 births. More than half of those (about 70%) are female, and most are stillborn.

Conjoined twins aren't linked to any specific race or maternal age. Having conjoined twins doesn't affect your ability to have a normal pregnancy later.

A pregnancy involving conjoined twins is considered high-risk. It may cause serious complications and requires a team of professionals to manage. If the babies survive, they must be delivered by C-section.

How are conjoined twins diagnosed?

Doctors usually diagnose a conjoined twin pregnancy during a regular ultrasound in the first or second trimester of your pregnancy. But conjoined twins can be detected as early as 7-12 weeks of pregnancy.

If your ultrasound does confirm conjoined twins, your doctor will suggest follow-up testing, including an MRI and an echocardiogram.

  • MRI: It shows where your twins are connected and which organs they share.
  • Echocardiogram: It uses sound waves to create images of your babies' hearts. It shows if their hearts are connected in any way.

Symptoms of conjoined twins

You won't be able to tell you're carrying conjoined twins from your pregnancy symptoms. You may experience symptoms similar to those of a typical twin pregnancy. These include:

  • Tiredness, nausea, and vomiting early in your pregnancy. While most expectant mothers have these symptoms, if you're carrying multiples, these symptoms are more severe.
  • Your uterus may grow faster than if you carried just one baby.

Monitoring conjoined twin pregnancy

If your doctors confirm you are pregnant with conjoined twins, they'll monitor your pregnancy closely. You'll likely have a team of doctors for you and your babies. The team may include:

  • Maternal and fetal medicine specialist 
  • Pediatric surgeons (cardiovascular, plastic, and orthopedic)
  • Pediatric urologist
  • Pediatric cardiologist
  • Neonatologist
  • Social worker
  • Palliative care specialist

You'll likely have monthly ultrasounds to check your babies' growth, amniotic fluid levels, and signs of heart problems. Blood tests can also check your conjoined twins' blood flow.

At around 32 weeks, if all is going well, you'll get tests more often, maybe twice a week. If it's hard to get separate heart readings on each baby, your doctor may do a test called a biophysical profile to monitor your babies. They may also give you steroids to help your babies' lungs develop and get ready for early delivery.

You'll have an MRI in the third trimester, which will give your health care team a better view of your babies' bodies to help them plan for the delivery. The team may even build a life-size 3D model from the MRI scans. 

But many conjoined twin pregnancies don't make it that far. Conjoined twin pregnancies are extremely high-risk, and many end in miscarriage. 

Some parents may decide to terminate the pregnancy after carefully considering the risks, including how long the babies might survive, their quality of life, and the care they might require. Ever since the U.S. Supreme Court overruled Roe v. Wade, abortions are no longer a constitutional right under federal law. At least 12 states now ban all abortions. In 29 other states, abortions are legal only up to a certain number of weeks of pregnancy.

Conjoined twins birth

Most conjoined twins need to be delivered by C-section to lower your risk of complications. There's a high chance that conjoined twins wouldn't be able to pass through your birth canal, which could cause your uterus to rupture. A C-section also lowers the risk of injury to your babies, as well.

There will probably be a lot of medical professionals in the delivery room with you, and each twin will have their own medical team, led by a neonatologist. The medical teams will rehearse the delivery beforehand to make sure everything goes smoothly.

Once your babies are delivered, their cords will be quickly clamped to prevent blood circulation problems. Their team of doctors will examine them and let you know the next steps. If one twin is stillborn or unlikely to live, emergency surgery may be necessary. If not, your doctors may want to wait some months to examine your babies' growth.

Not all conjoined twins are attached together in the same way. They're classified depending on how and where they're attached to each other. Nearly 75% of all conjoined twins are joined at the chest or abdomen. But conjoined twins can be joined at any of these sites:

Chest (thoracopagus). The twins face each other and are connected in the rib cage area. They usually share a single heart and might also share a liver. They tend to have four arms and four legs. 

Abdomen (omphalopagus). These twins face each other and are connected below the sternum near the belly button. They might share a liver, a biliary tract, and part of their upper digestive tract. They usually have four arms and four legs.

Lower back (pyopagus). The twins are joined back to back and may share genital organs, a urinary tract, a lower digestive tract, and the bones and nerves of the lower body. They might have two or three legs, though this is uncommon.

Pelvis (parapagus). These twins are fused side-by-side, usually at the pelvis. A rare form called dicephalic parapagus is when twins have two heads and brains on one torso.

Head (craniopagus). These twins are connected at the top of the skull. Their brains are usually connected, though not always.

Less common sites where conjoined twins can be connected include:

Spine(rachipagus). Such twins are very rare and are joined at their back, along the entire length of their spine.

Pelvis (ischiopagus). Twins are either face to face or end to end and joined at the pelvis. They may share the lower gastrointestinal (GI) tract, liver, genitals, and urinary organs. The twins could each have two legs or share two or three legs. 

Head and chest (cephalopagus). These twins are one of the rarest. They're joined at the head and are fused all the way to the belly button. They often share a brain and may share all their vital organs.

 
 

 

Not all conjoined twins are good candidates for surgery to separate them. For twins who share vital organs such as a heart, surgery isn't an option. Doctors usually wait three months to a year after the birth of conjoined twins to decide on separation. This is to give the twins time to grow strong enough for the operation. This also gives doctors enough time to understand the twins' bodies before surgery.

In some cases, conjoined twins need emergency separation surgery. If one twin dies or has a life-threatening medical event, the surgery might save the other twin's life.

Elective separation surgery is very complicated. You may need to travel to a hospital where the operation can be done. Depending on what your twins need after the surgery, you may even have to relocate to be near the twins' medical team. Children will need specialized follow-up care, including physical and occupational therapy.

Some conjoined twins don't have separation surgery and grow into adulthood still conjoined. They usually need special accommodations for transportation and housing as they grow up. Some pairs go on to live independent lives and have careers and relationships.

Unfortunately, the outlook for conjoined twins isn't very good. More than 50% of conjoined twin pregnancies result in miscarriage or stillbirth. About 35% of those who are delivered die within 24 hours of birth because their organs can't support them. 

Of the conjoined twins who do survive, only 25% live long enough to be candidates for surgery. Also, only about 60% of surgically separated twins survive.

Still, there is good news. More conjoined twins are surviving today than in the past, thanks to improvements in imaging studies and surgical techniques.

Conjoined twins are twins who are physically attached to one another. They can be joined at almost any body part, but they are most often connected at the chest or abdomen. The majority of conjoined twins are stillborn or die within the first 24 hours after birth. The 25% who survive beyond that may be candidates for separation surgery, and some go on to lead independent lives after separation.

What happens when one conjoined twin dies?

It depends on how they are joined. Usually, the other twin dies shortly after the first because the twins often share vital organs or blood circulation. If one dies shortly after birth (or is stillborn), doctors may be able to do emergency surgery and separate them. 

How many conjoined twins are there in the world?

Estimates range from 1 out of every 50,000 to 200,000 live births, making conjoined twins a rare condition. 

Can conjoined twins be different genders?

Generally, no. That's because they come from a single fertilized egg that didn't split completely. There have been some rare cases where identical twins have been of different sexes. Such cases happen when one twin loses a Y chromosome after the zygote splits. Remember that sex and gender don't always mean the same thing. Your sex refers to your chromosomes and genitalia. Your gender is how you identify socially and culturally, which may not be the same as your sex. Lori and George Schappell were the oldest pair of conjoined twins when they died in 2024 at age 62. Both were assigned female at birth, but in 2007, George came out as transgender.