Chromosomal and Genetic Conditions

Conjoined Twins

What are conjoined twins?

When the bodies of twins connect, they are called conjoined twins. Conjoined twins begin as a single fertilized egg. Usually, a single egg develops into 1 baby. Sometimes a single egg divides in half 1 to 2 weeks after it’s fertilized. This creates a set of identical twins.

The exact cause of conjoined twinning is not known. There are 2 theories:

  • The egg divides late and does not divide completely.
  • The egg divides completely but then 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” connecting their livers.

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

  • Vital organs, like 1 heart
  • Many structures, like several parts of the digestive, genital and urinary systems
  • A large segment of the body, like all of the lower body
  • Part of the brain and skull

The twins’ symptoms may depend in large part on which structures they share.

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 all conjoined twins are in this group. These twins are face to face. In about 75% of cases, these twins share a heart. Twins joined at the chest may also share a liver, biliary tract (which carries bile from the liver to the small intestine) and upper digestive tract (esophagus, stomach and small intestine).
  • Joined from the breastbone to the waist, called omphalopagus or xiphopagus. These twins are face to face. They may share a liver, biliary tract and upper digestive tract. About 34% of conjoined twins are in this group.
  • Joined at the sacrum (lower back) and buttock area, called pygopagus. These twins are back to back. They may share part of the lower digestive tract (large intestine, rectum and anus) and parts of the skeleton, nervous system and genitals. About 18% 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 a liver and biliary tract, part of the upper and all of the lower digestive tract, the genital and urinary systems and part of the skeleton. About 6% of conjoined twins are in this group.
  • Joined at the head, called craniopagus. These twins may share a skull, brain and other parts of the 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. Seattle Children’s Prenatal Diagnosis and Treatment Program helps families get ready for the birth and plan the steps ahead.

Usually, prenatal care includes scheduling delivery by cesarean section (C-section) a month before the twins’ due date. This is because a vaginal birth is too hard for the mother and babies.

Among the conjoined twins who survive more than a day after birth, some live for days, weeks, months or years while conjoined. Some live into adulthood still physically connected to each other. Seattle Children’s provides complete care to meet all the healthcare needs of conjoined twins from birth through childhood and into the early adult years.

Some conjoined twins 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 many cases, both twins survive after surgery. In some cases, only one survives — often because the other doesn’t have the organs they need or the organs are not well developed. Sometimes neither child survives.

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). The experts at Seattle Children’s Prenatal Diagnosis and Treatment Program can perform these studies for you and explain what the images show. You can consult with Seattle Children’s 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 the healthcare team prepare for the diagnostic tests and provide the treatment the twins may need once they are born.

These babies will need a C-section for delivery. 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, extensive 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 with a team of doctors to talk about your twins, including the benefits and risks of having surgery and 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

Seattle Children’s has a large amount of experience separating conjoined twins. Dr. John Waldhausen led a team of 30 healthcare providers to perform 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.

Read more about expert treatment for conjoined twins at Seattle Children’s.

Contact Us

If you have questions about conjoined twins treatment or prenatal diagnosis, call our General and Thoracic Surgery Department at 206-987-2794, extension 4, or our Prenatal Diagnosis and Treatment Program at 206-987-5629.