Conjoined or Siamese Twins: Types, Causes, and Management

Conjoined twins are a rare phenomenon when twin siblings are born physically joined and share one or more body parts. The life expectancy and quality of life depend on the level of fusion and vitality of fused organs.

Key takeaways:

Conjoined twins are also known as Siamese twins after the famous conjoined twin Bunker brothers born in Siam (now Thailand) in 1811. This type of twin pregnancy is rare, with an estimated frequency of 1.5 cases per 100,000 births worldwide. The survival rate of conjoined pregnancies is 7.5%. Unfortunately, 60% of these cases result in stillbirth, and 88% of live births do not survive after birth.

How do twin pregnancies happen?

Fraternal twin pregnancies occur when two sperm cells fertilize two separate eggs. As two individuals formed from different parental cells, their physical features and genders are independent.

Sometimes cells derived from one egg and one sperm may split and continue as two or more fetuses. Since the "start point" is the same for these siblings, these twins will be physically and genetically identical. Hence, identical twins always have the same gender.

Joined or incompletely separated?

The scientific world has not reached an agreement on this question. While some scientists theorize that the incomplete splitting of embryonic cells causes this phenomenon, others argue that all types of conjoined twins happen by joining two embryos.

If we agree with the first theory, the splitting day will determine circumstances for twins' growth.

Day of splitting after fertilizationGrowth conditions
1 to 3 daysEach twin will have individual placentas and amniotic sacs (dichorionic diamniotic)
4 to 8 daysShared placenta with separate amniotic sacs (monochorionic diamniotic)
8 to 12 daysShared placenta and amniotic sacs (monochorionic monoamniotic)
13 days or laterShared placenta, amniotic sacs and body parts (conjoined twins)

Interestingly, 70% of conjoined twins are females. Female twins are commonly joined in the chest, while male twins are joined in the abdomen and pelvis.

Current scientific literature is limited to offering any genetic or environmental factor to understand the etiology of the condition.

Types of conjoined twins

Conjoined twins may have fused body segments and shared organs on different levels. The location of joining affects the viability of twins and their survival chances after separation surgeries.

  • Parapagus twins. This is the most common type of conjoined twins and is observed in 28% of conjoined pregnancies. In this type, twins joined at the abdomen, pelvis, and lower limbs. These siblings have two faces with separate or fused skulls. Parapagus-type twins are viable but not likely to survive upon separation.
  • Thoracopagus twins. The second most common type was observed in 19% of conjoined pregnancies. In this type, siblings are joined from the chest to the umbilicus (belly button). Although babies will have two heads and separate limbs, their vital organs, such as the liver, lungs, and heart, will be shared. This type of conjoined twin is rarely viable and not likely to survive the separation.
  • Omphalopagus twins. Observed in 18% of conjoined twin cases. In this type, twins are fused from the chest bone (sternum) to the umbilicus and share only abdominal organs, such as the liver and intestines. They are compatible with life and could be separated with surgery.
  • Ischiopagus twins. Observed in 11% of cases. The twins are joined on the end parts of the trunk and face each other. They share genitalia, pelvic and lower digestive organs. This type is also viable and can be surgically separated.
  • Cephalopagus twins. Observed in 11% of conjoined pregnancies. The twins are joined from the head to the umbilicus. These siblings have joined head, chest, and abdomen. They are not compatible with life and cannot be separated with surgery.
  • Other types. In other rare types of conjoined cases, siblings have fused skull (craniopagus), vertebra (rachipagus), the end part of the vertebra (pyopagus), or the lower end of the chest bone sternum (xiphopagus). These types also have a better prognosis, and babies are expected to survive separation surgeries. Interestingly, the famous Bunker brothers and the first recorded separation surgery were xiphopagus twins.

Parasitic twin. Is a condition observed in conjoined twining when one of the siblings develops normally, and the other harbors severe defects. This extremely rare pathology is commonly associated with rachipagus cases.

Diagnosis of conjoined twins

The diagnosis of conjoined monoamniotic pregnancies can be confirmed during prenatal screening.

Ultrasonography (USG) performed in the first trimester can successfully diagnose the number of embryos. If a twin pregnancy is confirmed, USG is also helpful in confirming the number of amniotic sacs. In monoamniotic pregnancies, twins grow one shared compartment.

Conjoined twinning happens in monoamniotic pregnancies. Thus, if only one sac is detected, next, fetuses are observed in relation to each other. Fused body parts could be visualized as early as 7 to 12 weeks. Conjoined fetuses generally have congenital organ abnormalities. Such defects, shared or separate formation of vital organs, can be assessed during the second-trimester detailed USG.

Fetal echocardiography evaluates heart chambers, aorta, and other cardiovascular structures. It should be remembered that twins with a shared heart are not eligible for separation surgery.

Fetal magnetic resonance imaging (MRI) provides more detailed information on fused segments, shared organs, and congenital defects.

In 50% of cases, the pregnancy fluid increases at the later stages of pregnancy (polyhydramnios). USG imaging also assists with the evaluation of the fluid level.

Management of conjoined twins

Despite being a rare condition, if diagnosed, these pregnancies require a careful evaluation, family counseling, and surgical preplanning.

Pregnancy follow-up

Conjoined twin pregnancies require close monitoring in high-risk pregnancy units throughout the pregnancy.

Management of such a complex situation should involve a multidisciplinary team with maternal-fetal medicine, pediatric, plastic, and neurosurgery, cardiology and cardiovascular surgery, urology, neonatology, and radiology specialists.

A detailed evaluation by these experts will provide more insights into the organ structures, their functional capacities, survival prognosis, and separation surgical options.

Delivery of conjoined twins

Vaginal birth is possible in these cases. In the literature, there are some cases of undiagnosed conjoined twins which were delivered vaginally. However, it should be remembered that it carries a high risk of complications, including dystocia (obstruction of labor), rupture of the uterus, and fetal death.

Vaginal delivery can be implemented in nonviable twins. It also could be considered in the second-trimester deliveries, as these fetuses are smaller than mature ones.

Cesarean section is the delivery method of choice in conjoined twins. The recommended delivery time is 35 weeks. Postponed delivery may increase the risks associated with polyhydramnios or stillbirth.

During the cesarean section, the abdomen and uterus are opened vertically. This technique allows the womb to open more widely and reduces the risk of traumatic delivery.

A comprehensive assessment is required after delivery to decide on the separation surgery.

Separation surgery

The first recorded separation surgery with the survival of both siblings was performed on the xiphopagus sisters by Dr. Johannes Fatio in 1689 in Switzerland. The first long-term surviving separated craniopagus twin sisters were operated by Dr. Harold Varis in 1955 in Chicago.

Surgical techniques and experiences advanced in the following decades. Nowadays, we can find dozens of survival stories of separated twins worldwide. However, even today, the decision of separation surgery is a complex ethical, legal, and medical notion.

Experts present different arguments on how separation might introduce a moral dilemma and violate the autonomy of individuals. As famously known from the Jodie and Mary case, the legal aspect of the separation surgery is another discussion topic.

After delivery, separation surgery options are not the same for all cases:

  • Do not separate. If surgery is considered life-threatening or might result in extreme disability, these cases might be left joined. The famous story of the conjoined Abby and Brittany sisters is an example of happy and successful lives without a separation surgery.
  • Emergency separation. In cases when one sibling dies or a life-threatening condition develops, an emergency separation might be required.
  • Delayed separation. After extensive evaluation and family counseling, the surgery generally is planned for 6 to 12 months of the babies' lives if a separation decision is made.


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