The Siamese

The Siamese

Siamese siblings are twins from a single pregnancy. Both embryos are from the same egg but the separation in two of the fertilized egg is not done completely. The two fetuses will then develop while remaining welded to each other.
This malformation occurs in monoamniotic twins, that is to say in fetuses that have developed in a single pocket of amniotic fluid in utero. When the twins are viable, delivery is by caesarean section.

The incidence of Siamese twins is very low, around 1 in 100,000. Siamese twin sisters are more frequent than Siamese brothers: 3 to 1. The largest proportion of girls is found in Siamese thoracopages (united by the thorax) whereas we find a majority of boys in the parapages (lateral union by the pelvis). There is no explanation for these differences.
There is no risk of recurrence (having Siamese does not pose an additional risk of having others).
No factors were significantly associated with Siamese twins (maternal age, inbreeding, genetic and environmental factors, etc.). In particular, there are no chromosomal abnormalities.

Siamese twins surgery

When the union between the two is not important, the anomaly is reversible. Surgery can be performed to separate the two twins. An operation all the more effective if no vital organ is merged.

Some twins are indeed connected by the arm, the head, the thorax while others are connected by the brain or the cardiac device. The operation becomes more delicate in this case because the blood vessels of the brain and heart irrigate the body of the two infants.

Most often, ultrasound diagnosis of fused twins leads to termination of pregnancy. If the pregnancy is over and one of the twins dies or is dying, surgical separation is attempted urgently. Under these conditions the operative mortality of the survivor is 70 to 80%.

When the separation is decided by the doctors and the parents, it is done at the age of 2 to 4 months. This time is necessary to allow the twins to stabilize, to make a very precise assessment, and to the surgical teams to prepare themselves.

The surgical operation to separate Siamese can be very complex, requiring multidisciplinary teams, including 2 teams of anesthetists (one for each twin). It is a heavy surgery, difficult and long, and risky for patients. In almost 20% of cases, the result of the operation resulted in the death of one or both twins. It is for this reason that today, doctors do not hesitate to refuse to carry out the operation and even to stop an operation in progress if the surgeons perceive the too great risk for the patients.

In this field, there were, in the 2000s, only a dozen surgical teams in the world capable of attempting these interventions.
A female pathology

The history of medicine to date has 200 Siamese twins separated in the world. This malformation concerns a pregnancy in 100,000 and 1% of twin pregnancies. In nine out of 10 cases, they are binoculars. Most Siamese children do not survive.

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