Chromosomal and Genetic Conditions

Conjoined Twins

What Are Conjoined Twins?

Twins whose bodies are connected are called conjoined twins.

Conjoined twins begin as a single fertilized egg. Usually a single fertilized egg develops into a single baby. Sometimes a single egg divides in half during the first one to two weeks after it's fertilized. This creates a set of identical twins.

The exact cause of conjoined twinning is not known. There are two theories. One is that the egg divides late and does not divide completely. The other is that the egg divides completely but then fuses (joins) back together.

The connection between the twins' bodies may be fairly simple. They may share only a small amount of tissue, and both children may have all the organs and other structures they need. For example, the twins may be joined at the belly with a "bridge" that connects their livers.

Usually the connection is more complex, and sometimes it is very complex. The children may share:

  • Vital organs, like one heart
  • Many structures, like several parts of their digestive, genital and urinary systems
  • A large segment of their body, like all of their lower body
  • Part of the brain and skull

Types of conjoined twins

Doctors group conjoined twins based on where they are joined.

  • Joined at the chest, called thoracopagus. This is the most common type. About 40% of conjoined twins are in this group. These twins are face to face. In about 75% of cases, these twins share one heart. Twins joined at the chest may also share their liver, biliary tract (which carries bile from the liver to the small intestine) and upper digestive tract.
  • Joined from the breastbone to the waist, called omphalopagus or xiphopagus. These twins are face to face. They may share their liver, biliary tract and upper digestive tract. About 35% of conjoined twins are in this group.
  • Joined at the sacrum and buttock area, called pygopagus. These twins are back to back. They may share part of their lower digestive tract and parts of their skeleton, nervous system and genitals. About 20% of conjoined twins are in this group.
  • Joined in the pelvic area, possibly up to the breastbone, called ischiopagus. These twins may be oriented to each other in different ways. In general, they partly face each other. They may share their liver and biliary tract, part of their upper and all of their lower digestive tract, their genital and urinary systems and part of their skeleton. About 6% of conjoined twins are in this group.
  • Joined at the head, called craniopagus. These twins may share their skull, brain and other parts of their nervous system. About 2% of conjoined twins are in this group.

Outlook for Conjoined Twins

Most sets of conjoined twins do not survive because their organs cannot support them. About 40% of conjoined twins are not alive when they are delivered (stillborn). About 35% die within a day after they are born.


Even so, more conjoined twins survive now than in the past. Advances in imaging, surgical techniques and anesthesia have helped improve chances for survival. We also have tools to detect their condition before birth (prenatally). This means their families and doctors can plan their early care before they are born.

Usually this includes scheduling delivery by cesarean section (C-section) a month before their due date. This is because a vaginal birth is too hard for mother and babies.

Among the conjoined twins who survive more than a day after birth, some continue to live for days, weeks, months or years while conjoined. Some live into adulthood still physically connected to each other.

Some have surgery to be separated, usually in the first year of life. The success of this surgery depends on many factors, mainly where the twins are connected and which structures they share. In some cases, both twins survive after surgery. In some cases, only one survives, or neither does.

Making decisions

Each set of conjoined twins is unique. Decisions about their care are complex and often hard for their families and their healthcare providers. The chance for survival is one important factor in these decisions. Another important factor is the likely quality of the twins' lives if they stay conjoined or are separated.

Parents who find out they are pregnant with conjoined twins can have imaging studies to learn more about their twins' condition. Usually these are ultrasound and MRI (magnetic resonance imaging). They can consult with doctors who care for conjoined twins about what to expect, including whether separation might be possible. Some parents continue their pregnancy, and some decide to end their pregnancy.

For parents continuing their pregnancy, doctors will suggest careful monitoring and planning. This helps their healthcare team prepare for the diagnostic tests and provide the treatment the twins may need once they are born.

After delivery, the twins will likely need many types of imaging studies and tests to learn as much as possible about how they are connected. This information will help the family and healthcare team decide how to care for the twins.

Conjoined Twins at Seattle Children's

At Seattle Children's, we have the knowledge, experience and facilities to perform separation surgeries - and to provide care for conjoined twins who cannot be separated or whose family decides not to separate them.

Separation is not the only choice for these children. Some parents may choose not to have this surgery even though their children could be separated. You will meet a team of doctors to help go over the pros and cons of surgery as well as the pros and cons of not being separated. Our goal is to help you understand your twins' health and all your options so you can make the choices that are best for your family.

Separation surgery

Though conjoined twinning is rare, Seattle Children's has a large amount of recent experience. As one example, in October 2001, a team of 30 healthcare providers at Children's led by Dr. John Waldhausen performed a successful 31-hour surgery to separate conjoined twins Kathleen and Charity Lincoln. Worldwide only about 250 separation surgeries have ever been successful, meaning at least one twin survived over the long term, according to the American Pediatric Surgical Association.

Team approach

Every family who comes to Seattle Children's has a team of people to care for their child before, during and after surgery. For families with conjoined twins, this team approach is especially important. Many types of healthcare providers are needed to assess the twins' condition, and plan and carry out their treatment.

Along with your children's surgeons, you are connected with doctors trained to care for new babies with complex problems (neonatologists), nurses, dietitians, child life specialists, social workers and others. We work together to meet all of your children's health needs and help your family through this experience.

Focus on children

Since 1907, Seattle Children's has been treating children only. Our team members are trained in their fields and also in meeting the unique needs of children. For example, the doctors who give your children anesthesia are board-certified in pediatric anesthesiology. This means they have extra years of training in how to take care of kids. Our child life specialists know how to help children understand their illnesses and treatments in ways that make sense for their age. Our expertise in pediatrics truly makes a difference for our patients and families.