Medically reviewed on March 7, 2018
Conjoined twins are two babies who are born physically connected to each other.
Conjoined twins develop when an early embryo only partially separates to form two individuals. Although two fetuses will develop from this embryo, they will remain physically connected — most often at the chest, abdomen or pelvis. Conjoined twins may also share one or more internal organs.
Many conjoined twins die in the womb (stillborn) or die shortly after birth. Some surviving conjoined twins can be surgically separated. The success of surgery depends on where the twins are joined and how many and which organs are shared, as well as the experience and skill of the surgical team.
Conjoined twins may be joined at one of several places. These conjoined twins are joined at the chest (thoracopagus). They have separate hearts but share other organs.
There are no specific signs or symptoms that indicate a conjoined twin pregnancy. As with other twin pregnancies, the uterus may grow faster than with a single fetus, and there may be more fatigue, nausea and vomiting early in the pregnancy. Conjoined twins can be diagnosed early in the pregnancy using standard ultrasound.
How twins are joined
Conjoined twins are typically classified according to where they're joined, usually at matching sites, and sometimes at more than one site. They sometimes share organs or other parts of their bodies. The specific anatomy of each pair of conjoined twins is unique.
Conjoined twins may be joined at any of these sites:
- Chest. Thoracopagus (thor-uh-KOP-uh-gus) twins are joined face to face at the chest. They often have a shared heart and may also share one liver and upper intestine. This is one of the most common sites of conjoined twins.
- Abdomen. Omphalopagus (om-fuh-LOP-uh-gus) twins are joined near the bellybutton. Many omphalopagus twins share the liver, and some share the lower part of the small intestine (ileum) and colon. They generally do not share a heart.
- Base of spine. Pygopagus (pie-GOP-uh-gus) twins are commonly joined back to back at the base of the spine and the buttocks. Some pygopagus twins share the lower gastrointestinal tract, and a few share the genital and urinary organs.
- Length of spine. Rachipagus (ray-KIP-uh-gus), also called rachiopagus (ray-kee-OP-uh-gus), twins are joined back to back along the length of the spine. This type is very rare.
- Pelvis. Ischiopagus (is-kee-OP-uh-gus) twins are joined at the pelvis, either face to face or end to end. Many ischiopagus twins share the lower gastrointestinal tract, as well as the liver and genital and urinary tract organs. Each twin may have two legs or, less commonly, the twins share two or three legs.
- Trunk. Parapagus (pa-RAP-uh-gus) twins are joined side to side at the pelvis and part or all of the abdomen and chest, but with separate heads. The twins can have two, three or four arms and two or three legs.
- Head. Craniopagus (kray-nee-OP-uh-gus) twins are joined at the back, top or side of the head, but not the face. Craniopagus twins share a portion of the skull. But their brains are usually separate, though they may share some brain tissue.
- Head and chest. Cephalopagus (sef-uh-LOP-uh-gus) twins are joined at the face and upper body. The faces are on opposite sides of a single shared head, and they share a brain. These twins rarely survive.
In rare cases, twins may be asymmetrically conjoined, with one twin smaller and less fully formed than the other.
Identical twins (monozygotic twins) occur when a single fertilized egg splits and develops into two individuals. Eight to 12 days after conception, the embryonic layers that will split to form monozygotic twins begin to develop into specific organs and structures.
It's believed that when the embryo splits later than this — usually between 13 and 15 days after conception — separation stops before the process is complete, and the resulting twins are conjoined.
An alternative theory suggests that two separate embryos may somehow fuse together in early development.
What might cause either scenario to occur is unknown.
Because conjoined twins are so rare, and the cause isn't clear, it's unknown what might make some couples more likely to have conjoined twins.
Conjoined twins must be delivered by cesarean section. Many conjoined twins are stillborn or die shortly after birth. Not all surviving twins are candidates for separation surgery. The success of surgery depends on where the twins are joined and how many and which organs are shared, as well as the experience and skill of the surgical team.
Conjoined twins can be diagnosed using standard ultrasound as early as the end of the first trimester. More-detailed ultrasounds and echocardiograms can be used about halfway through pregnancy to better determine the extent of the twins' connection and the functioning of their organs.
If an ultrasound detects conjoined twins, a magnetic resonance imaging (MRI) scan may be done. The MRI may provide greater detail about where the conjoined twins are connected and which organs they share.
Treatment of conjoined twins depends on their unique circumstances — their health, where they're joined, and whether they share organs or other vital structures.
Monitoring during pregnancy
If you're carrying conjoined twins, you should be closely monitored throughout your pregnancy. You'll likely be referred to a maternal and fetal medicine doctor who specializes in high-risk pregnancy. You may also be referred to other specialists such as pediatric surgeons, pediatric cardiologists and neonatologists.
Your doctors and others on your health care team learn as much as possible about your twins' anatomy, functional capabilities and prognosis to form a treatment plan for your twins.
A surgical delivery (C-section) is planned ahead of time, often two to four weeks before your due date.
After your conjoined twins are born, they're fully evaluated. With this information, you and your health care team members can make decisions regarding their care and whether separation surgery is appropriate.
Separation surgery is an elective procedure done two to four months after birth. Sometimes an emergency separation may be needed if one of the twins dies, develops a life-threatening condition or threatens the survival of the other twin.
Many complex factors must be considered as part of the decision to pursue separation surgery. Each set of conjoined twins presents a unique set of considerations due to variations in anatomy. Questions may include:
- Do the twins share vital organs, such as the heart?
- Are the twins healthy enough to withstand separation surgery?
- What are the odds of successful separation?
- What type of reconstructive surgery might be needed for each twin after successful separation?
- What functional support will be needed after separation?
- What issues would the twins face if left conjoined?
Recent advances in prenatal imaging, critical care and anesthetic care have improved outcomes in separation surgery.
If surgery isn't an option
If separation surgery isn't possible or if you decide not to pursue the surgery, your team can help you meet the medical care needs of your twins.
If the circumstances are grave, medical comfort care — such as nutrition, fluids, human touch and pain relief — is provided.
Coping and support
Learning that your unborn twins have a major medical issue or life-threatening condition can be devastating. As a parent, you struggle with difficult decisions for your conjoined twins and the uncertain future. Outcomes can be difficult to determine, and conjoined twins who survive sometimes face tremendous obstacles.
Because conjoined twins are rare, it may be difficult to find supportive resources. Ask your health care team if medical social workers or counselors are available to help. Depending on your needs, ask for information on organizations that support parents who have children with significant physical conditions or who have lost children.
Preparing for an appointment
If you're pregnant with conjoined twins, you'll be referred to a team of specialists to help guide you and create a treatment plan for your twins. Here's some information to help you get ready and what to expect from your doctor.
What you can do
Before your appointment:
- Consider taking a family member or friend along. Sometimes it can be hard to remember all the information provided during an appointment. Someone who comes with you may remember something that you missed or forgot.
- Make a list of questions to ask your doctor. List your questions from most to least important to make the most of your appointment time.
Examples of questions you may want to ask your doctor include:
- What tests do my twins need?
- Where are my twins joined and what, if any, body organs do they share?
- What is the best treatment plan?
- Is separation an option? What is the likely outcome with separation?
- What are the alternatives to the treatment approach that you're suggesting?
- What specialists should be involved in the health care team?
- Are there any other specialists I should meet with?
- Where can I find support for my family?
- Are there printed materials that I can have? What websites do you recommend?
- If I choose to have more children, is there a chance they may also be conjoined?
Don't hesitate to ask other questions during your appointment.
What to expect from your doctor
Your doctor and health care team will review your conjoined twins' tests and exam results and discuss options with you. Together with your health care team, you can make decisions for your twins' treatment and care.