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Identical (monozygotic) twins occur when a single fertilized egg splits and develops into two fetuses. In a few cases, the fertilized egg fails to separate completely. The babies that develop from this partial separation are called conjoined twins.

Unlike most identical twins, conjoined twins share the same amniotic fluid and placenta. They're also physically connected — most often at the head, chest or pelvis. Conjoined twins may even share one or more internal organs.

Many conjoined twins are stillborn or die shortly after birth. Some surviving conjoined twins can be surgically separated. The success of this surgery depends on where the twins are joined and how many organs are shared, as well as on the experience and skill of the surgical team.

The first sign of conjoined twins is the same as for any twin pregnancy: a rapidly growing uterus that's larger than for a single embryo. Mothers of twins may also have severe fatigue, nausea and vomiting early in the pregnancy.

How twins are joined
Conjoined twins are usually classified according to point(s) where they're joined. More than three dozen conjunctions have been identified. Some of the more common include:

• Thoracopagus. One of the most common of conjoined twins, thoracopagus twins are joined at the chest. They always have a shared heart and may also share one liver and upper intestine.
• Omphalopagus. Also common, omphalopagus 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.
• Pygopagus. Pygopagus twins are joined at the base of the spine. Some pygopagus twins share the lower gastrointestinal tract, and a few share the genital and urinary organs.
• Ischiopagus. Ischiopagus twins are joined at the pelvis. Many ischiopagus twins share the lower gastrointestinal tract, as well as the genital and urinary tract organs.
• Craniopagus. Craniopagus twins are joined at the head. Craniopagus twins share the skull. Some also share the cerebral cortex — the part of the brain that plays a central role in memory, language and perception.
• Parapagus. Parapagus twins are joined extensively from side to side.

Many conjoined twins have separate arms and legs, but in some cases, limbs are shared. In rare cases, twins may be asymmetrically conjoined, with one twin smaller and less fully formed than the other (parasitic twins).

Identical twins occur when a single fertilized egg splits and develops into two fetuses. The split normally happens during the first 12 days after conception. When the fertilized egg splits later than this — usually between 13 to 14 days after 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.

What causes the egg to separate or why this sometimes occurs later than normal isn't clear, but experts suspect that it may be due to a number of genetic and environmental factors.

Conjoined twins are rare — about 10 occur in every million births. It's not known if anything makes some couples more likely to have conjoined twins. It is known, however, that conjoined twins occur more often in India and Africa than they do in the United States.

Up to 50 percent of conjoined twins develop excessive amniotic fluid (polyhydramnios). Too much amniotic fluid may increase the risk of serious pregnancy complications, such as premature rupture of the membranes, preterm labor, problems with the umbilical cord and stillbirth.

Most often, conjoined twins result in tragedy for the pregnancy. Vaginal delivery is rarely possible. About 40 percent of conjoined twins are stillborn. Of conjoined twins born alive, less than half survive long enough to be candidates for separation surgery.

Parents of conjoined twins must make profoundly difficult decisions, and some are best made early in the pregnancy. The use of high-resolution and transvaginal ultrasound — an imaging method in which the ultrasound probe is placed in the vagina — makes it possible to diagnose conjoined twins in the first trimester. False-positive results can occur before 10 weeks, however, when identical twins who share an amniotic sac (monoamniotic twins) may appear conjoined.

Treatment of conjoined twins varies greatly depending on the circumstances.

Many parents make the difficult decision to end the pregnancy. Prognosis and quality of life issues weigh heavily in the decision, as well as the likelihood of successful separation. If the babies share a heart or brain, for example, separation surgery may not be possible.

If the parents choose to continue the pregnancy, mother and babies will be closely monitored throughout the pregnancy. A surgical (C-section) delivery is planned ahead of time, often two to four weeks before the due date.

After the conjoined twins are born, the parents and doctors must decide whether to attempt separation surgery. 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. More often, however, separation surgery is an elective procedure done two to four months after birth. The survival rate for elective separation is 80 percent — much higher than the 29 percent survival rate for emergency separation. But although survival rates have improved considerably, surgical separation is still relatively rare.

Again, many factors weigh heavily in the decision to pursue separation surgery. Do the twins share any vital organs? 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 issues would the twins face if left conjoined?

If separation surgery isn't possible or the parents decide not to pursue the surgery, comfort care is provided as needed.

Nothing is harder than for expectant parents to learn that their unborn child has a life-threatening condition. In the case of conjoined twins, this is doubly difficult — not just because there are two babies involved, but also because the children who survive face tremendous obstacles, and the parents must grapple with tremendous decisions.

Conjoined twins are rare, and few public resources exist specifically for conjoined twins or their families. But there are a number of organizations that support parents who have lost children or who have children with grave physical problems. Your medical team can put you in touch with some of these groups as well as with medical social workers and counselors.