The embryology of conjoined twins

By Anastasia Alenova

Occurring once in 50,000 births, conjoined twins are a rare and abnormal form of monozygotic twins. Conjoined twins are classified into seven types based on their site of attachment, followed by the suffix “pagus” which means “fixed” in Greek (O’Neill et al., 1988; Mian et al., 2017). The fascination for conjoined twins has been present for centuries, with books and engravings dating back to medieval times found on the subject (Hartman, 2006). Despite being seen as a spectacle in the past, certain conjoined twins, such as Chang-Eng Bunker, managed to live fulfilling lives, even getting married and having healthy children (Mian et al., 2017). Little is known about the embryology of conjoined twins, but better understanding of the subject may lead to better survival rates and quality of life. 

The mechanism leading to conjoined twins is not understood, but there are two theories, fission and fusion, present in the literature. The first theory is that of “fission”. Normally, identical twins arise from the fertilised ovum undergoing fission. The twinning event, or cleavage of the embryo, can occur at three different stages (Spencer, 1992). The earliest possibility is at the morula, or early blastocyst, stage. In this case, each embryo has a separate placenta. If cleavage occurs later at the inner cell mass within the blastocyst, the embryos each have a separate placenta. In both cases presented, each embryo has a separate amniotic cavity (Kaufman, 2004; Mian et al. 2017). However, the embryonic splitting can occur at an even later stage, right at or even after the development of the primitive streak. In this scenario, both embryos share a common placenta and amniotic cavity. It is in this third case scenario of monochorionic and monoamniotic twinning that conjoined twins may arise (Kaufman, 2004; Mian et al. 2017). When the “twinning” stimulus occurs at a later stage, it can cause incomplete splitting of the embryonic axis (Spencer, 1992).

The second theory is termed “fusion” and believes that conjoined twins arise from fusion of the embryonic discs at the periphery. Post-fertilisation, the embryo develops into a flat disc of continuously growing and developing cells. Prior to four weeks, duplication is ongoing and possible fusion sites are exposed and vulnerable. According to this hypothesis, conjoined twins are a result of notochords, precursors of vertebral development, supposed to be separate, but too close to develop independently (Spencer, 1992; Mian et al., 2017). 

Once conjoined, the anatomical structures of the growing embryos can adapt. This adaptation depends on the extent and time of fusion and can be divided into two types. The first type, division and diversion, involves division of the midline structures in half and joining to the homologous half. The second type, aplasia, involves sagittal and lateral anatomical structures joining asymmetrically leading to one structure overtaking the other, which is caused to atrophy (Mian et al. 2017).  This adaptation is far from perfect and conjoined twins are more prone to congenital anomalies and often have severe morphological problems. Depending on the extent of the overlap of both embryos, different organ systems may be involved (Spencer, 1992; Kaufman, 2004). Conjoined twins can be identified as soon as the early second trimester, by identifying key characteristics such as no separating membrane between twins, inability to separate foetal bodies or skin contours and constant head position. Accurate prenatal analysis is crucial to ensure a correct diagnosis of the shared anatomy and concurrent malformations, and appropriate counselling for the affected families. The evaluation should consist of ultrasonography, magnetic resonance imaging and echocardiograms, to gain better understanding of the anatomy of the shared organs. In rare cases, early evaluation may recognise a case where separation at birth is required in order to save the life of a twin, thus maximising chances of survival. Early diagnosis also allows to opt for termination, as 60% of conjoined twins are stillborn, or continuation with a fair understanding of the risks. Conjoined twins are usually delivered preterm and by caesarean (O’Neill et al., 1988; MacKenzie et al., 2002). 

Separation surgery is complex and requires a large team of specialists. It may not always be recommended, as survival and a fulfilling life is possible conjoined. However, surgery may be necessary if the life of one of the twins is at risk, for example if the other twin has severe congestive heart failure. In less urgent cases, it is best to delay the separation until maturity, about 6-12 months of age. This reduces the risks of anaesthesia, confirms anatomic relationships and helps detect any previously unrecognised congenital anomalies (O’Neill et al., 1988; MacKenzie et al., 2002). The feasibility and success of separation surgeries heavily depend on the type and extent of the fusion. In cases of thoracopagus twins, joined at the thorax, cardiac union often prevents surgery as heart chambers and great vessels may be intertwined. For twins joined at the head, cephalopagus, separation is not technically possible and should not be attempted as it would end in death of one or both twins (Spencer, 1992). In craniopagus twins, when the neural system is shared, surgery is extremely complex and ill-advised, since despite possibly having separate brains, components of the skull, cerebral connections and vasculature may be shared, meaning that post-surgery effects are hard to predict (Mian et al., 2017).   Often, the survival of the twins requires extensive reconstruction surgery, as many organs and structures are shared. This is feasible, and there have been reported cases of twins surviving past 30 years of age while retaining normal function or various organ systems (Kaufman, 2004). 

The success of surgery and quality of life of conjoined twins is a case by case scenario. Separation surgery may also pose heavy ethical issues when having to choose the life of one twin to ensure survival, or a better life, of the other one. Great medical advances have been made, ensuring better survival of conjoined twins and higher rates of successful separation. However, conjoined twins can still lead a fulfilling and healthy life without being separated.


OʼNeill, J. A., Holcomb, G. W., Schnaufer, L., Templeton, J. M., Bishop, H. C., Ross, A. J., Duckett, J. W., Norwood, W. I., Ziegler, M. M. & Koop, C. E. (1988) Surgical Experience with Thirteen Conjoined Twins. Annals of Surgery. 208 (3), 299-312. Available from: doi: 10.1097/00000658-198809000-00007. 

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 (New York, N.Y.). 30 (3), 385-396. Available from: doi: 10.1002/ca.22839. 

Hartman L. (2006) From “Monsters” to modern medical miracles [online] U.S. National Library of Medicine. Available at: [Accessed 15th Oct. 2020]

Spencer, R. (1992) Conjoined twins: Theoretical embryologic basis. Teratology (Philadelphia). 45 (6), 591-602. Available from: Available from: doi: 10.1002/tera.1420450604. 

Kaufman, M. (2004) The embryology of conjoined twins. Child’s Nervous System. 20 (8), 508-525. Available from: Available from: doi: 10.1007/s00381-004-0985-4. 

MacKenzie, T. C., Crombleholme, T. M., Johnson, M. P., Schnaufer, L., Flake, A. W., Hedrick, H. L., Howell, L. J. & Adzick, N. S. (2002) The natural history of prenatally diagnosed conjoined twins. Journal of Pediatric Surgery. 37 (3), 303-309. Available from: Available from: doi: 10.1053/jpsu.2002.30830. 

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