Conjoined twins

Last revised by Dr Yahya Baba on 15 Nov 2020

Conjoined twin pregnancy is a rare occurrence resulting from the failure of a zygote to separate completely after 13 days 11. This results in the twins being physically joined.

The prevalence of conjoined twins ranges from 1:50,000 to 1:200,000. They are more common in parts of Southeast Asia and Africa with prevalence rates as high as 1:14,000 to 1:25,000. There is a recognized female predisposition (F: M of approximately 3:1).

Conjoined twins are monozygotic, monoamniotic, and monochorionic (MCMA) (see multifetal pregnancy) and result due to a failure of normal complete separation of the embryonic plate from an incomplete delayed division of the inner cell mass. This is thought to occur around 13-17 days of gestation.

Conjoined twins are classified according to the most prominent site of interconnection

If more than one area is connected the terms are combined, e.g. thoraco-omphalopagus (thoracic and abdominal fusion).

Other descriptive terms include:

There is a higher incidence of congenital malformations in conjoined twins (10-20%) which are unrelated to the point of fusion such include

Thus identification of a dividing membrane or two placentas excludes the diagnosis. Definitive sonographic features will depend on the type of fusion.

General features include:

  • lack of a separating inter-twin membrane
  • non-separable skin contours with an inability to separate the fetal bodies
  • detection of other anomalies in a twin gestation
  • solitary umbilical cord with more than 3 vessels present
  • both fetal heads persistently at the same level
  • backward flexion of the cervical spine (due to the fact that most conjoined twins are fused ventrally and face each other
  • bibreech or less commonly, bicephalic presentation
  • constant relative fetal positions

The prognosis for conjoined twins, in general, is quite poor. Approximately 40-60% of conjoined twins are stillborn and almost 35% of live births do not survive beyond 24 hours. Of those who do survive, surgical separation is sometimes possible (but with higher failure rates if performed within the first 3 weeks 9). Surgical separation, in general, is in most cases very challenging with high mortality, depending on the complexity of shared structures. Of those with thoracopagus, ~75% have extensively joined hearts which in turn preclude a successful separation.

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Cases and figures

  • Figure 1
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  • Case 1: parapagus
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  • Case 2: thoracopagus
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  • Case 3: cephalothoracophagus
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  • Figure 2
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  • Case 4: ischiopagus
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