Conjoined Twins

Author: Vanessa Mora

Editor: Nicholas Jo


Conjoined twins, also known as Siamese twins, is a rare condition where genetically identical twins fuse during fertilization (Mian et al., 2017). Fusing causes twins to be born physically connected, where multiple different physical attachment sites remain after birth (Mian et al., 2017).



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



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:

  • Elevated levels of nausea and vomiting
  • Higher hormone concentration (Mitsuda et al., 2019).

Possible Attachments

  • Abdomen: Some organs are mutually shared, excluding the heart
  • Upper chest: Organs are shared, including the heart
  • Head and towards the navel: Organs and two hearts are shared
  • Lower abdomen: Genitals are mutually shared
  • On the sides: Some have separated torsos but remain attached, some share a torso but have two heads, and others may share one torso and one head but have two faces
  • Lower back: Spinal cord and anus are shared (Mathew et al., 2017).


Risk Factors

Risk factors that increase the likelihood of acquiring amblyopia include:

  • Consumption of antifungal medication during pregnancy
  • Exposure to radiation during pregnancy (Mutchinick et al., 2011).


Medical Diagnosis

As early as 12 weeks of pregnancy, an ultrasound is performed to diagnose conjoined twin pregnancy (Mian et al., 2017). An ultrasound detects the position of shared organs, and attachment location between fetuses (Mian et al., 2017). Additionally, an MRI scan may be performed to provide a more accurate view of the fetus (Mathew et al., 2017).



Following diagnosis, it is advised that all conjoined twin births are Cesarean sections (Mian et al., 2017). The treatment of conjoined twins is separation surgery, but how the procedure proceeds depend on their attachment.

  • Immediate surgery: Required if the 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 surgery: Performed if both twin vitals are stable. This can be done at three months of age (Mian et al., 2017).



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.

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.

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