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How rare are conjoined twins? [top] [next]
- Extremely rare. Although conceived perhaps once in every 40,000 pregnancies, many abort spontaneously or are terminated, while almost all the rest are stillborn or die within 24 hours of birth. Live births occur around once in every 200,000 of all live births.
- Worldwide, there are probably only five unseparated sets of teenage or adult twins living today.
- There are also four or five sets of conjoined babies at any time, either awaiting separation or who are not separable and will die, usually within 18 months of birth.
- There could be as many as 50 complete sets of separated twins alive today, and more single twins who are sole survivors of separation.
Why do they form? [top] [next]
Conjoined twins originate from one fertilised egg. They are identical, usually female, and share one placenta. No one knows why they form. Genetic and environmental factors have been explored, but because of the scarcity of twins to study, it is hard to reach any conclusions. There are two theories about what happens in the womb:
- The fertilised egg, which should have split into the embryos of identical twins, does not split properly.
- Identical twin embryos start to develop separately, and fuse together later on in pregnancy.
Detection before birth [top] [next]
In the West, conjoined twins are usually picked up during routine ante-natal ultrasound scanning. Conjoined twins have been detected as early as nine weeks' gestation, but diagnosis in the third trimester of pregnancy is more reliable. Some types, for example those joined at the head, are harder to distinguish on sonograms than others.
Types of conjoined twins [top]
Classification tends to depend on the point at which the twins are joined, the Greek suffix, 'pagos' meaning 'that which is fixed'. Six basic classifications are listed here, but each set of twins is uniquely joined and over 30 separate types have been identified.
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Omphalopagus/thoracopagus
Joined at the chest (omphalopagus), sometimes also with a shared heart (thoracopagus). The majority — around 40% — are joined in this way. When the heart is not involved, as with Chang and Eng, separation prospects are good. Twins with joined hearts have a very poor prognosis for survival, and many, like the Lawler twins featured in the programme, die at birth. Two successful separations have recently taken place. The Lewis twins survived unseparated with conjoined hearts until the age of six.
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Pygopagus
Positioned back-to-back, and joined at the base of the spine. Affects around 20% of twins. This form is not usually life-threatening unless major organs are joined, but may present complications in splitting the spinal cord, as with Esther and Stella. The 19th-century McKoy and Godina twins were both pygopagus.
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