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What are Conjoined Twins?
When the bodies of twins connect, they’re known as conjoined twins. Conjoined twins begin as one fertilized egg. Usually, one egg develops into one baby. Typically one egg divides in half 1 to 2 weeks when it’s fertilized. This creates a group of identical twins.
The exact cause of conjoined twinning isn’t known. There are two theories:
- The egg divides late and doesn’t divide fully.
- The egg divides fully but then joins back together.
The connection between the twins’ bodies may be fairly easy. They will share only a small quantity of tissue, and both children could have all the organs and other structures they need. For instance, the twins is also joined at the belly with a “bridge” connecting their livers.
Usually, the connection is more advanced, and typically it’s very complex. The kids may share:
- Vital organs, like one heart
- Many structures, like many 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 bone
The twins’ symptoms could rely in large part on which structures they share.
Symptoms of Conjoined Twins
There aren’t any specific signs or symptoms that indicate a conjoined twin pregnancy. Like other twin pregnancies, the uterus could grow quicker than with a single fetus, and there is also more fatigue, nausea and vomiting early in the pregnancy. Conjoined twins may be diagnosed early in the pregnancy using normal ultrasound.
How twins are joined
Conjoined twins are generally classified according to where they are joined, typically at matching sites, and sometimes at more than one site. They often share organs or other parts of their bodies. The precise anatomy of each pair of conjoined twins is unique.
Conjoined twins is also joined at any of these sites:
- Chest. Thoracopagus (thor-uh-KOP-uh-gus) twins are joined face to face at the chest. They typically have a shared heart and should also share one liver and upper intestine. This is often one of the most common sites of conjoined twins.
- Abdomen. Omphalopagus (om-fuh-LOP-uh-gus) twins are joined close to the bellybutton. Several omphalopagus twins share the liver, and some share the lower a part of the small intestine (ileum) and colon. They generally don’t 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 known as 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 finish. Several ischiopagus twins share the lower gastrointestinal tract, as well as the liver and genital and urinary tract organs. Every twin could have 2 legs or, less usually, the twins share 2 or 3 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, however with separate heads. The twins will have 2, 3 or four arms and 2 or 3 legs.
- Head. Craniopagus (kray-nee-OP-uh-gus) twins are joined at the back, top or side of the head, however not the face. Craniopagus twins share a portion of the bone. However their brains are typically separate, though they will 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 seldom survive.
In rare cases, twins may be asymmetrically conjoined, with one twin smaller and less fully formed than the other.
Causes of Conjoined Twins
Identical twins (monozygotic twins) occur once a single fertilized egg splits and develops into 2 fetuses. The split normally happens during the first 12 days when conception. It’s believed that once the fertilized egg splits later than this typically between 13 and 14 days when conception separation stops before the process is complete, and the resulting twins are conjoined. The degree to which the egg splits and when the split happens determines how and where the twins will be joined.
But, another theory suggests that 2 separate embryos might somehow fuse together in early development.
What would possibly cause either scenario to occur is unknown.