Conjoined twins are monochorionic-monoamniotic twins fused at any portion of their body as a result of an incomplete division of the embryonic disk, which occurs after 13th day of conception . The Term “conjoined” is a misnomer because researchers consider the pathogenesis of this condition resulting from a failure of the complete separation rather than from fusion of the twins .
The conjoined twins have an estimated incidence 0.1-0.2:10.000 births [3,4]. The incidence appears to be independent of maternal age, race, parity and heredity.
Conjoined twins are generally classified according to the area of the bodies where the fusion takes place and the involvement of the internal organs. The symmetrical and equal forms, in which the twins have equal or nearly equal duplication of structures, are called duplicata completa. When there is an unequal duplication of structures, the twins are called duplicata incompleta and this category includes the most severe types of conjoined twins in which just a few organs are duplicated. The most frequent varieties of conjoined twins are thoracopagus (40 to 74 %), omphalopagus (10 to 30%), pygopagus (18%), ischiopagus (6%), and craniopagus (1 to 6%).
The minimal sonographic criterion for the prenatal diagnosis of conjoined twins is the visualization of the fused portion of the bodies of monozygotic-monoamniotic twins, also the following sonographic features can be recognized: (a) inability to separate fetal bodies, (b) absence of the interamniotic membrane between the twins, (c) the heads and the bodies of the twins are seen at the same level 5, (d) unusual extension of the spines, (e) unusual proximity of the extremities, (f) bifid appearance of the first trimester fetal pole, (g) presence of a single heart and (h) the presence of abnormal number of vessels in the umbilical cord.
Modern imaging modalities, such as 3D, 4D, and rapid fetal MRI can be used in order to support the diagnosis, and exclude possible pitfalls in the sonographic diagnosis of the conjoined fetuses.
Conjoined twins have an unique presentation, but a few differential diagnoses could include lymphangioma, teratoma or cystic hygroma.
Many anomalies can be associated with this condition. Cardiac defects are the most common association (20 to 30 %); thus echocardiography is recommended in all cases. Neural tube defects and mid-line fusion defects, orofacial clefts, imperforated anus, and diaphragmatic hernia are also frequently seen. Polyhydramnios is observed in 50 to 75 % of the cases. In contrast, karyotype abnormalities are virtually absent in conjoined fetuses.
Accurate antenatal classification allows early counseling with parents so that the options of pregnancy termination or near term cesarean delivery in order to minimize maternal morbidity can be discussed.
Most of the conjoined twins are born prematurely, 40 % are stillborn, and 35 % die within the first 24 hours . Among the survivors, the prognosis and the attempt in surgical separation will depend on the type of conjunction, degree of involvement of the shared organs, and the presence of associated anomalies. The most ominous prognosis is among those twins who share the liver and/or the heart. In the presence of a shared heart, separation is only attempted if two normal hearts coexist in a single pericardium.
Obstetrician play a pivotal role in prenatal counseling and in coordinating meetings between parents and members of the interdisciplinary medical team needed in the management of these complex cases.
1. Wigglesworth JS, Singer DB, eds. Textbook of Fetal and Perinatal Pathology. Cambridge, MA: Blacwell Scientific Publications, 1991:221-262.
2. Spencer R. Conjoined Twins: Theoretical embryologic basis. Teratology. 1992;45:591-602.
3. Hanson JW. Incidence of Conjoined Twining. Lancet 1975; 2: 1257.
4. Edmons LD, Layde PM. Conjoined Twins in the United States 1970 – 1977. Teratology 1982; 25:301-308.
5. Koontz WL, Herbert WN, Seeds JW, et al. Ultrasonography in the antepartum diagnosis of conjoined twins. A report of two cases. J Reprod Med. 1983;28:627-630.