Twins have long been a source of interest and fascination.

Fraternal (dizygotic) twins occur when two separate eggs are fertilized by two different sperm. Identical (monozygotic) twins occur when a single fertilized egg splits and develops into two fetuses. In a few cases, identical twins fail to completely separate into two individuals. These babies are known as conjoined twins.

Conjoined twins may develop in as many as one in 50,000 pregnancies, but conjoined twins account for only one in about 250,000 live births. Female conjoined twins outnumber male conjoined twins at a rate of three to one.

Conjoined twins are most often joined at the chest, head or pelvis. In some cases, conjoined twins share one or more internal organs. At times, conjoined twins can be surgically separated. The success of separation surgery depends in part on where the twins are joined and how many organs are shared.

Conjoined twins are classified by how their bodies are joined together. There are many possibilities. For example:

• Thoracopagus twins. Thoracopagus twins are joined at the chest. Thoracopagus twins may share one liver, and most share the sac around the heart (pericardium). Some also share the heart muscle and the upper intestine.
• Omphalopagus twins. 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 the colon.
• Pygopagus twins. 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 twins. 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 twins. 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 twins. Parapagus twins are joined extensively from side to side.

Various types of conjoined twins have separate arms and legs. In some cases, however, limbs are shared as well.

Identical twins occur when a single fertilized egg splits and develops into two fetuses. The split normally happens during the first two weeks after conception. If the fertilized egg splits later, the twins will be conjoined. The degree to which the fertilized egg splits and when the split happens determines how the twins will be joined. The cause of delayed splitting isn't known.

Early clues to a twin pregnancy may be a uterus that's larger than normal or more than one fetal heartbeat. A suspected twin pregnancy can be confirmed with an ultrasound as early as the 12th week. If the twins are conjoined, the ultrasound may reveal a single placenta and umbilical cord or the lack of separating membranes in the placenta. Separate bodies or skin contours may be difficult to distinguish. By the 20th week, detailed ultrasounds and other imaging tests may reveal the anatomy of any shared organs, including the heart.

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, 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.

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.

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. Throughout the world, there have been cases of conjoined twins who have lived happy, rewarding lives.