Conjoined Twins Types, and Separation Surgery

What are conjoined twins?

 Picture of Conjoined Twins
Picture of Conjoined Twins

Conjoined twins are a rare type of twins in which part of the twins' bodies are fused together. When this happens, the fusion occurs between the same body parts (for example, head to head, chest to chest). The location and extent of the fusion of conjoined twins can vary. Congenital anomalies (birth defects) are always present in the case of conjoined twins.

What are the signs and symptoms of a conjoined twin pregnancy?

As with twins or other normal (non-conjoined) multiple gestations (for example, triplets), the uterus may grow faster than with a singleton gestation (carrying one baby). There may also be excessive nausea and other symptoms of a typical twin or multiple pregnancy. However, there are no specific signs or symptoms to suggest that a pregnant woman is carrying conjoined twins.

How common are conjoined twins?

  • Conjoined twins are estimated to occur in around 1.5 per 100,000 births worldwide.
  • Females are affected more often than males; about 75% of conjoined twins are female.

What are the types of conjoined twins?

The medical terms used to describe the types of conjoined twins are based on the specific location of the fusions:

  • Thoracopagus or xiphopagus twins are joined facing each other at the chest. They may have a shared heart and other organs.
  • Omphalopagus twins are joined near the navel (belly button), and may share abdominal organs.
  • Craniopagus twins are joined at some area of the skull, typically the back, top, or side of the head. They share a portion of the skull but usually have separate brains.
  • Parapagus twins are joined at the trunk, appearing side to side at the hips along with joining of parts of the abdomen and chest. The may have varying numbers of arms and legs but have two separate heads. Pygopagus twins commonly are joined at the back, at the lower spine and buttocks.
  • Ischiopagus twins are joined at the pelvis and may be joined facing each other or side to side. They may share parts of the gastrointestinal system, genital tract, or urinary tract.
  • Rachipagus or rachiopagus twins are rare. They are joined at the back along the length of the spine.
  • Cephalopagus twins are joined at the head and part of the chest. They have a single head with one brain and faces on the opposite sides. Cephalopagus twins typically do not survive.

The term "Siamese twins" was used in the past to refer to conjoined twins based on a famous pair of twins from the 19th century, but this term is no longer used in the medical literature.

What causes conjoined twins to form together?

  • Conjoined twins are thought to develop when an embryo does not fully separate to form two individual bodies.
  • The reason for this lack of separation is not understood.

What are risk factors for having conjoined twins?

  • Because the cause of conjoined twins is not understood and because this condition is so rare, there are no known risk factors for having a conjoined twin pregnancy.
  • If a woman has one conjoined twin pregnancy, she is not believed to be at increased risk for having conjoined twins in her next pregnancy.

How are conjoined twins delivered and treated? What is separation surgery?

Conjoined twins are delivered by Cesarean section (C-section), typically at a somewhat earlier gestational age than full-term pregnancies.

After birth, the decision as to whether or not to attempt surgical separation depends on many factors, such as the:

  • Extent to which the twins share vital organs
  • Types of reconstructive surgery that will be needed
  • Likelihood that separation will be successful

These questions must be weighed against the outlook, and/or issues the twins would face, and the complications they may experience if not surgically separated.

What is the prognosis and life expectancy for conjoined twins?

The outlook for conjoined twins depends upon the extent and location of the fusion of body parts. In many pregnancies, conjoined twins die in utero or do not survive long after birth. Stillbirth affects about 40% of sets of conjoined twins.

In other situations, the conjoined twins survive, and surgical separation may be successful in some females. The birth defects may be severe and life threatening, particularly after surgical separation, for some conjoined twins. Pregnancy risks for women carrying conjoined twins include polyhydramnios (excessive amniotic fluid), preterm birth, and fetal death (stillbirth).

Can having conjoined twins be prevented?

Because there are no known risk factors for conjoined twins, this condition cannot be prevented.


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Kamal, K, MD, et al. Conjoined Twins. Medscape. Updated: May 08, 2018.