Conjoined Twins

Author: Vanessa Mora

Editor: Nicholas Jo


Conjoined twins, also known as Siamese twins, is a rare condition where genetically identical monoamniotic and monochorionic twins fuse during fertilization in the uterus (Mian et al., 2017). This rare condition occurs in 1 out of 50,000-100,000 live births, with a range of 40-60% being stillborn, 35% surviving for 24 hours, and an overall survival rate of 18% (Kapoor et al., 2012). Conjoined twins are classified based on physical attachment after birth (Mian et al., 2017). 

The first ever successful conjoined twin separation was in 1953 with pygopagus twins (Carlson et al., 2018). Since then, conjoined twin separation has become more common, with a survival rate of 64% among 167 cases (Carlson et al., 2018). The classifications with the highest mortality are seen in thoracopagus and craniopagus twins with around 51% mortality, while pygopagus and ischiopagus have the lowest mortality rate of around 23% and 19% (Carlson et al., 2018).



Conjoined Twins, Siamese Twins, Identical Twins, Monozygotic Fission, Monozygotic Fusion, Monochorionic, Monoamniotic



The classification system of conjoined twins is based upon the point of attachment after birth. The possible attachments points are:

  • Omphalopagus: Attachment is located on the abdomen, starting from the lower chest towards the groin (Mian et al., 2017). Gastrointestinal organs are mutually shared, excluding the heart (Mathew et al., 2017). 
  • Thoracopagus: Attachment is located on the upper chest and joined facing each other (Mian et al., 2017). Any cardiovascular body part may be shared, including the heart (Mathew et al., 2017).  
  • Cephalopagus: Attachment is located on the head towards the navel (Mathew et al., 2017). Gastrointestinal organs and two hearts are mutually shared (Mian et al., 2017). 
  • Ischiopagus: Attachment is predominantly located on the lower abdomen. However, sometimes attachment may begin from the head (Mian et al., 2017). Pelvic bones and genitalia are mutually shared (Mathew et al., 2017). 
  • Parapagus: Attachment is located on the sides. Some have separated torsos but remain attached, some share a torso but have two heads, and others may share one torso, one head but have two faces (Mathew et al., 2017). 
  • Pyopagus: Attachment is located in the lower back and coccyx. The spinal cord and anus are mutually shared (Mian et al., 2017). 



Conjoined twins arise during a twin pregnancy, when the embryo’s separation on the 13th-15th day of fertilization is incomplete, or when the embryo fuses in early development (Kaufman, 2004). 



Conjoined twin pregnancy symptoms are the same as a regular twin pregnancy (Mitsuda et al., 2019). Symptoms that indicate a twin pregnancy compared to a single child pregnancy are: 

  • Higher maternal hormone concentrations (androgens, estrogens, insulin-like growth factor (IGF)-1, IGF binding protein (BP)-3, and prolactin) (Houghton et al., 2019)
  • Higher levels of Human chorionic gonadotropin (hCG) (Korevaar et al., 2015)
  • Elevated levels of nausea and vomiting (Mitsuda et al., 2019).


Risk Factors 

Consumption of griseofulvin, an antifungal medication, during pregnancy has been linked to two cases of conjoined twins, as well as potential congenital disabilities (Mutchinick et al., 2011). Griseofulvin has the ability to cross the placental barrier and is classified as a teratogen that could potentially cause birth defects (Mutchinick et al., 2011). However, prescription of griseofulvin is uncommon as population-based studies conducted on 22,853 pregnancy outcomes with birth defects reported only 0.06% of cases had griseofulvin exposure (Mutchinick et al., 2011).

Consuming substances such as tobacco, alcohol and drugs during pregnancy have not been related to conjoined twin pregnancy (Sharma et al., 2010). Additionally, 5 cases of conjoined twins have been reported after exposure to chronic radiation doses during pregnancy (Mutchinick et al.,2011). Ethnicity may also increase the chance of producing conjoined twins, where India has 29%, and South Africa has 16% of conjoined twin cases (Sharma et al., 2010). 



Pathological Features

As early as 12 weeks of pregnancy until the end of the first trimester, an ultrasound is performed to diagnose conjoined twin pregnancy (Mian et al., 2017). Ultrasound scanning allows physicians to detect the position of shared organs, attachment location and areas of body/skin contact between fetuses (Mian et al., 2017). It can also determine the presence of significant complications, such as polyhydramnios, which is found in half of all conjoined twin cases. (Mian et al., 2017). Additionally, fetal echocardiography and 3D-MRI scans help define tissue characterization and provide a more accurate view of fetal abnormalities (Mathew et al., 2017).  


Treatment Protocol  

Non-Pharmacological Treatment Protocol

Following diagnosis, a decision about surgical procedures can be made after birth. All conjoined births are advised to be done as Caesarian sections (Mian et al., 2017). 

  • Immediate separation surgery: Required if the location of attachment and sharing of organs is life-threatening to either twin. This procedure has the potential to save one or both twins (Mian et al., 2017). 
  • Delayed separation surgery: Performed if both twin vitals are stable. This can be done after birth, at three months of age (Mian et al., 2017). 


Case Study

An example of a successful operation involved an ischiopagus twin, where they shared a single terminal ileum and a colon (Carlson et al., 2018). The rest of the anatomical features of the twins are as follows: 4 kidneys, 2 bladdres where 1 ureter form each twin emptying to the bladder of the opposite twin (Carlson et al., 2018). The twins then shared a single anus with 4 ovaries and 2 uteri (Carlson et al., 2018). The most complex problem with separation is insufficient soft tissue coverage post operation which can be caused by tissue edema (Carlson et al., 2018).

The use of 3-D modelling as well as tissue expansion has been useful especially with this case study, and closure may not have been possible post surgery without these tools (Carlson et al., 2018). Negative pressure therapy and using allograft material can help solve small areas of exposure persistent after surgery but come with their own individual risks and complications (Carlson et al., 2018). Overall, the ischiopagus twins made a full recovery but their lower extremity function still continues to be monitored, with pelvic reconstructive surgery a possibility in the future (Carlson et al., 2018).


Articles on Misdiagnosis

Herkiloğlu, D., Baksu, B., & O. Pekin. (2016). Early prenatal diagnosis of thoraco-omphalopagus twins at ten weeks of gestation by ultrasound. Turkish Journal of Obstetrics and Gynecology, 13(2), 106-108. DOI: 10.42744/tjod.89814. 

Osmanağaoğlu, M.A., Aran, T., Güven, S., Kart, C., Özdemir, O., & H. Bozkaya. (2011). Thoracopagus conjoined twins: a case report. International Scholarly Research Notices, 2011. DOI: 10.5402/2011/238360. 

Takrouney, M., et al. (2020). Conjoined twins: a report of four cases. International Journal of Surgery Case Reports, 73, 289-293. DOI: 10.1016/j.ijscr.2020.06.072. 



  • Carlson, T.L., Daugherty, R., Miller, A., Gbulie, U.B., & R. Wallace. (2018). Successful Separation of Conjoined Twins: The Contemporary Experience and Historic Review in Memphis. Annals of Plastic Surgery, 80(6S): S333-S339. DOI:10.1097/SAP.0000000000001342.
  • Houghton, L. C., Lauria, M., Maas, P., Stanczyk, F. Z., Hoover, R. N., & Troisi, R. (2019). Circulating maternal and umbilical cord steroid hormone and insulin-like growth factor concentrations in twin and singleton pregnancies. Journal of developmental origins of health and disease, 10(2), 232–236. DOI: 10.1017/S2040174418000697.
  • Kapoor, M., Sachdev, N., & Agrawal, M. (2012). Conjoined Twins. The Journal of Obstetrics and Gynecology of India, 63(1), 70-71. DOI: 10.1007/s13224-012-0138-8
  • Kaufman, M. (2004). The embryology of conjoined twins. Child’s Nervous System, 20(8-9). DOI: 10.1007/s00381-004-0985-4.
  • Korevaar, T. I., Steegers, E. A., de Rijke, Y. B., Schalekamp-Timmermans, S., Visser, W. E., Hofman, A., Jaddoe, V. W., Tiemeier, H., Visser, T. J., Medici, M., & Peeters, R. P. (2015). Reference ranges and determinants of total hCG levels during pregnancy: the Generation R Study. European journal of epidemiology, 30(9), 1057–1066. DOI: 10.1007/s10654-015-0039-0
  • Mathew, R. P., Francis, S., Basti, R. S., Suresh, H. B., Rajarathnam, A., Cunha, P. D., & Rao, S. V. (2017).  Conjoined twins – role of imaging and recent advances. Journal of Ultrasonography, 17(71), 259-266. DOI: 10.15557/JoU.2017.0038
  • Mian, A., Gabra, N. I., Sharma, T., Topale, N., Gielecki, J., Tubbs, R. S., & Loukas, M. (2017). Conjoined twins: From conception to separation, a review. Clinical Anatomy, 30(3), 385-396. DOI: 10.1002/ca.22839.
  • Mitsuda, N., Eitoku, M., Maeda, N., Fujieda, M., & Suganuma, N. (2019). Severity of Nausea and Vomiting in Singleton and Twin Pregnancies in Relation to Fetal Sex: The Japan Environment and Children’s Study (JECS). Journal of Epidemiology, 29(9), 340-346. DOI:  10.2188/jea.JE20180059.
  • Mutchinick, O. M., Luna-Muñoz, L., Amar, E., Bakker, M. K., Clementi, M., Cocchi, G., da Graça Dutra, M., Feldkamp, M. L., & D. Landau et al. (2011). Conjoined twins: a worldwide collaborative epidemiological study of the International Clearinghouse for Birth Defects Surveillance and Research. American journal of medical genetics. Part C, Seminars in medical genetics, 157C(4), 274–287. DOI:  10.1002/ajmg.c.30321.
  • Sharma, G., Mobin, S. S., Lypka, M., & Urata, M. (2010). Heteropagus (parasitic) twins: A review. Journal of Pediatric Surgery, 45(12), 2454-2463. DOI: 10.1016/j.jpedsurg.2010.07.002.

Print this Article

Mini Disclaimer

User discretion: The material presented here is for your information and to be used for your reference only. Our volunteers have sourced this information from reputable sources and made sure it meets the quality standards at MARI. The information presented is not intended to be used as a diagnostic tool, a basis upon which to make diagnostic decisions, or to substitute the advice of a medical or healthcare professional. 

If you are a patient experiencing any of the listed symptoms, please consult a healthcare provider for specialized care and follow the instructions provided for you by your doctor. If you are in the US and would like advice on a possible case leading to errors in medicine, please reach out to our MARI Consultation team.

We appreciate your feedback!

Thank you for visiting our website. We are always looking for ways to improve. Please take a moment to tell us about your experience.