Conjoined twins are two babies who are born physically connected to each other. They 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. They may also share one or more internal organs. Though many conjoined twins are not alive when born (stillborn) or die shortly after birth, advances in surgery and technology have improved survival rates. Some surviving conjoined twins can have surgical separation. The success of surgery depends on where the twins are in joint and how many and which organs are share, as well as the experience and skill of the surgical team.

How twins are conjoined

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. Conjoined twins may be joined at any of these sites:

• Chest.
• Abdomen.
• Base of spine.
• Length of spine.
• Pelvis.
• Trunk.
• Head.
• Head and chest.

In rare cases, twins may be conjoint with one baby smaller and less fully form than the other. In extremely rare cases, one twin may have partial development within the other twin.

Causes of conjoined twins

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.

People believe 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.

Complications of conjoined

Pregnancy with conjoined twins is complex and greatly increases the risk of serious complications. Joined babies require surgical delivery by cesarean section (C-section) due to their anatomy.

As with twins, conjoint babies are likely to be born prematurely. One or both could be stillborn or die shortly after birth. Severe health issues for twins can occur immediately — such as trouble breathing or heart problems — and later in life, such as scoliosis, cerebral palsy or learning disabilities.

Potential complications depend on where the twins joint is, which organs or other parts of the body they share, and the expertise and experience of the health care team. When one expects these twins, the family and the health care team need to discuss in detail the possible complications and how to prepare for them.

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