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.