Development, types, possibility of operation… five questions to understand Siamese twins

5 questions sur les jumeaux siamois DR

Siamese twins are the result of a malformation during pregnancy: a late and incomplete division of the egg, the embryonic button.

Siamese children are twins who are born connected to each other by a body part. Although many of these fetuses die in utero, some live together. Sometimes, they can be separated during an often extremely complex surgical procedure.

On April 12, we learned of the death of Lori and George Schappell, confirmed by Guinness World Records as the oldest Siamese twins in the world. They were 62 years old and connected to each other at the skull.

How conjoined twins are formed ?

Siamese twins initially originate from a monozygotic, monochorionic and monoamniotic twin pregnancy. It is therefore the fertilization of an egg by a single sperm. The twins also share the same placenta and the same bag of waters.

Siamese twins are the result of a malformation during pregnancy: a late and incomplete division of the egg, the embryonic button. The two fetuses therefore remain united and can share all or part of an organ or a segment of the body. Thus, Siamese children are born joined to each other by a part of the body.

What are the types of conjoined twins ?

87% of Siamese twins are connected "front side", we speak of ventral junction. They are:

cephalopages: fusion of the top of the skull to the umbilicus (often dead in utero); thoracopages: fusion at the level of the thorax with the liver in common. Sometimes twins also share a common pericardium and heart; omphalopages: fusion of the lower part of the thorax at the umbilicus; xiphopages: connected by the xyphoid process (at the lower level of the sternum).

13% of Siamese twins have a “backward” junction, called dorsal. We then talk about twins:

craniopages: fusion at the level of the cranial vault without including the face. Twins can be connected at different levels, frontal, temporal or parietal. Lori and George Schappell, who shared 30% of their brains – the frontal and parietal lobes – were craniopagus twins; backbones: fusion through the middle of the spine; pygopages: fusion at the level of the sacrum, coccyx and perineum, with, in the majority of cases, the common rectum, anus and bladder.

How often are the births of Siamese newborns ?

The prevalence of conjoined twins is 1/50,000 to 200,000 births. 70% of these births concern girls.

This type of birth is extremely rare today in countries where prenatal ultrasound is the norm. We can in fact detect this malformation from the 12th week of pregnancy. "The prognosis depends on the site, the nature, the extension of the common organs as well as the association with other malformations", note The Pan African Medical Journal. Still according to the medical journal, approximately 40 to 60% of cases are stillborn and 35% do not survive after the first day.

When is the operation possible ?

Several options are possible. The health status of the children, the part of the body by which they are connected and the organs they have in common constitute the main decision criteria. Very often babies cannot be separated and are not viable. They then receive palliative care.

Babies may be viable but impossible to separate because the operation would be too dangerous or because the shared organs are vital.

"The most favorable conditions for a separation are met when the children do not share vital organs and are thus sufficiently stable after birth so that the operation could be postponed for a few months", note the Swiss Medical Editions on the Swiss medical forum site. Twins, with fusion in the lower part of the abdomen for example, can be separated more easily.

The success of these extremely complex operations is, in any case, always a feat. In France, two little Cameroonian girls were separated in 2019. They were connected by the liver and the base of the thorax.

Even more spectacular, the separation in Brazil in 2022 of little boys connected by the skull and who shared viral blood vessels together. Nine operations will have been necessary; including the last one at 11 p.m., to successfully separate them.

Why are we talking about conjoined twins ?

The term Siamese was taken from the true story of Chang and Eng Bunker, two linked twins born in Siam in the 19th century. They went to exhibit in the United States and then in Europe where they were given the name of the Siamese brothers.