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
- cephalopagus: face (rare)
- thoracopagus: thorax, commonest site (~70% 2,9)
- omphalopagus, xiphopagus: abdomen
- ischiopagus: pelvis ~5%
If more than one area is connected the terms are combined, e.g. thoraco-omphalopagus (thoracic and abdominal fusion).
Other descriptive terms include:
- diprosopus: two faces with one head and body
- dicephalus: two heads with one body
- syncephalus: facial fusion with or without thoracic fusion
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
Treatment and prognosis
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.
- 1. Kingston CA, Mchugh K, Kumaradevan J et-al. Imaging in the preoperative assessment of conjoined twins. Radiographics. 21 (5): 1187-208. Radiographics (full text) - Pubmed citation
- 2. Prayer D. Fetal MRI. Springer Verlag. (2010) ISBN:3540732705. Read it at Google Books - Find it at Amazon
- 3. Merz E, Bahlmann F. Ultrasound in obstetrics and gynecology. Thieme Medical Publishers. (2005) ISBN:1588901475. Read it at Google Books - Find it at Amazon
- 4. Gore RM, Filly RA, Parer JT. Sonographic antepartum diagnosis of conjoined twins. Its impact on obstetric management. JAMA. 1982;247 (24): 3351-3. - Pubmed citation
- 5. Preziosi P, Medici D, Maiorana A. Sonographic findings in cephalothoracopagus-conjoined twins. Pediatr Radiol. 1984;14 (6): 453-4. - Pubmed citation
- 6. Destephano CC, Meena M, Brown DL et-al. Sonographic diagnosis of conjoined diamniotic monochorionic twins. Am. J. Obstet. Gynecol. 2010;203 (6): e4-6. doi:10.1016/j.ajog.2010.09.007 - Pubmed citation
- 7. Lam YH, Sin SY, Lam C et-al. Prenatal sonographic diagnosis of conjoined twins in the first trimester: two case reports. Ultrasound Obstet Gynecol. 1998;11 (4): 289-91. doi:10.1046/j.1469-0705.1998.11040289.x - Pubmed citation
- 8. Basgül A, Kavak ZN, Sezen D et-al. Thoraco-omphalopagus conjoined twins detected at as early as 9 weeks of gestation: transvaginal two-dimensional ultrasound, color Doppler and fetoplacental Doppler velocity waveform findings. Fetal. Diagn. Ther. 2006;21 (5): 477-80. doi:10.1159/000093893 - Pubmed citation
- 9. Entezami M, Albig M, Knoll U et-al. Ultrasound Diagnosis of Fetal Anomalies. Thieme. (2003) ISBN:1588902129. Read it at Google Books - Find it at Amazon
- 10. Winkler N, Kennedy A, Byrne J et-al. The imaging spectrum of conjoined twins. Ultrasound Q. 2008;24 (4): 249-55. doi:10.1097/RUQ.0b013e31818c8858 - Pubmed citation
- 11. Ahmadi F, Keramat N, Haghighi H. Conjoined twin. (2012) International journal of fertility & sterility. 6 (2): 135-6. Pubmed