Conjoined twins are caused by two embryos that are joined together during fertilization, resulting in twins that are physically connected, most commonly at the abdomen, chest or head. Conjoined twins are rare. Complications can be reduced with close management from healthcare professionals.
When two babies are physically connected to each other at birth, they're called conjoined twins. Experts don’t know exactly what causes the condition. But it likely involves splitting or fusion of very early-stage embryos soon after fertilization. All conjoined twins are identical, and about two-thirds are assigned female at birth (AFAB).
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Conjoined twins are rare. It's estimated to occur only once in every 50,000 pregnancies. Because the conjoined anatomy is sometimes incompatible with life, many conjoined embryos will not survive. Up to 60% of conjoined twins are stillborn (not alive at birth) or die shortly after delivery.
Yes. Conjoined twins are always identical. They’re also more likely to be female.
There are two theories of what causes conjoined twins:
Both of these theories help explain how conjoined twins might form during the first 12-14 days of fertilization. But scientists still don’t know why this happens.
Conjoined twins are identified by specific terms. These terms describe the point of connection. Twins may be conjoined at the:
Conjoined twins can be identified as early as 12-weeks prenatally through imaging tests, including:
If imaging tests show that twins are conjoined, healthcare providers will try to predict potential complications. Then they’ll make a plan to address them at the time of birth.
It is very common for birthing parents to experience premature labor with conjoined twins. With early detection, the pregnancy can be managed to prevent pre-term labor and allow the twins to develop normally.
Delivery by C-section is the most common way to manage this situation.
Sometimes. Successful management usually involves several medical and surgical specialists. Sophisticated imaging techniques and other tests can help sort out the anatomy and physiology of both twins. The decision to attempt separation or not can bring up difficult ethical questions. These questions are usually discussed at length between the parents, the medical team, and often clergy and medical ethicists.
In some cases, it’s possible to separate conjoined twins. The procedure to separate twins always takes extensive planning by a team of highly experienced surgeons.
After conjoined twins are born, they are grouped into one of three categories:
Since the cause of conjoined twins is unknown, there’s no way to prevent conjoined twins from occurring during pregnancy.
Survival rates and quality of life for conjoined twins depend entirely on how the two babies are connected. Here are the survival rates for each type:
If separation surgery is attempted, only 60% of surgically separated conjoined twins survive. Healthcare providers can manage complications to improve the outlook for conjoined twins.
Depending on how the twins are joined, conjoined twins may live fully functioning lives thanks to advancements in surgical separation. Some conjoined twins who can't be separated can still enjoy a good quality of life if they’re closely monitored.
Conjoined twins can be identified as early as 12 weeks into your pregnancy. Your provider can usually confirm this around the 20-week mark.
A note from Cleveland Clinic
The very last thing parents want to hear is that there’s something unexpected going on with their unborn children. Since we don’t know why conjoined twins happen, the rare condition can’t be prevented. After diagnosis, you’ll want to start working with a team of highly qualified medical professionals who can monitor the development of your twins, help you develop a birth plan and start to discuss surgical interventions. Treatment of conjoined twins depends on where they’re conjoined and the extent of organ sharing. But there are surgical and medical options for many conjoined twins who survive birth. It’s normal to have a range of intense feelings, including fear, anger, guilt and sadness. Remember that your healthcare team is there to support you each step of the way.
Last reviewed by a Cleveland Clinic medical professional on 04/26/2022.
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