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What is Siamese twins?

21:55 - 4 May , 2021


It is a gestational alteration that gives rise to twins who are physically connected in different body areas, usually the abdomen, pelvis, or chest, often sharing one or several organs.

This anomaly arises when the embryo undergoes a partial division that creates the conditions for two fetuses to develop in parallel, which are normally stillborn or survive shortly after birth.

In cases the Siamese twins survive, medical and technological advances allow, in some cases, to separate them through surgeries whose viability will depend on the common organs and which part they are joined from.

Siamese twins’ classification depends on the body area where they are joined from:

  • Thoracopagus: joined by the thorax in the chest area, so they are face to face. They usually share the heart, liver, and part of the intestine.
  • Omphalopagus: joined in the abdomen on one side of the navel. They share the liver, colon, and ileum.
  • Pygopagus: joined at the bottom of the spine. They share part of the intestines and, in some cases, the urinary system and genitals.
  • Rachipagus: they are joined by the entire spine, being a rare case.
  • Ischiopagus: joined by the pelvis and sharing the liver, lower intestines, urinary tract, and genitalia. In certain cases, they can share lower limbs. 
  • Parapagus: they are joined by the trunk, being able to share upper and lower limbs. 
  • Craniopagus: they are joined by the back, front, side, or top of the head. Usually, each has their own brain, but they do share some nerve tissue.
  • Cephalopagus: joined at the head and chest. They have a single head and a single brain, so it is very difficult for them to survive.

Signs and symptoms Siamese twins

Although there are no clear symptoms that there is a Siamese twins pregnancy, there may be:

  • Larger uterus.
  • Exhaustion.
  • Persistent early nausea.
  • Constant early vomiting.

Diagnosis and treatment Siamese twins

Diagnosing the presence of Siamese twins is relatively simple after the first trimester of pregnancy through an ultrasound, echocardiogram, MRI, and imaging studies that allow to identify the fetuses connective area and their organic physiology.

Subsequently, a fetal echocardiogram is a useful tool to establish treatment during pregnancy and after birth.

Each case of Siamese twins is unique, so the treatment to be followed will depend on where they are joined from, which organs they share, their health conditions, and other aspects that allow determining the feasibility of separation surgery.

Continuous monitoring is required for the mother during pregnancy, as there are high risks. Therefore, it will be necessary for you to have at your disposal a specialized medical team that can provide optimal care to mothers and babies, made up of specialists in:

  • Neonatology.
  • Pediatric Surgery.
  • Pediatric urology.
  • Pediatric orthopedics.
  • Pediatric cardiology.

At the Pediatric Center, we provide specialized care to small patients from the moment of birth until they become adults, through our pediatric care, oncology, neurology, and cardiology services at the level of the best medical centers in the world.


  • kidshealth.org
  • topdoctors.es
  • healthychildren.org
  • mayoclinic.org
  • stanfordchildrens.org
  • medigraphic.com
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  • Gómez-Cadena JD, Sandoval-Martínez DK. Gemelos unidos (siameses): descripción de hallazgos anatomopatológicos. Ginecol Obstet Mex. 2018;86(12):823-830.
  • Quintana HD, Pérez ÁI, Barrios GE, et al. Gemelos siameses cefalotoracoonfalopago. Revista de Ciencias Médicas de la Habana. 2018;25(1):62-67.
  • Arteaga-Yañez JH, Sánchez-Montaño M, Negreros-Osuna JP, et al. Siameses toracópagos: reporte de caso. Anales de Radiología México. 2016;15(3):244-247.

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