Conjoined twins are defined as monochorionic monoamniotic twins fused at any portion of their body as a result of an incomplete division of the embryonic disk, occurring after the 13th day post conception. The term ‘conjoined’ is actually a misnomer, since most authors consider the pathogenesis of the condition to result from failure of complete separation, rather than fusion of twins. This is a rare condition and the reported frequency varies from 0.1 to 0.35:10,000 births. If stillbirths are excluded the estimate is 0.05:10,000. Females are more commonly affected with a female:male ratio of 1.6 to 3:1. No association with maternal age, race, parity or heredity has ever been observed. The recurrence risk is negligible. Most conjoined twins are born prematurely, 40% are stillborn, and 35% die within 24 hours. Among the survivors, the prognosis as well as attempts of 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 liver and or heart. Attempts of separation in cases of a common liver can be done as long as two biliary tracts are seen. In the presence of a shared heart, separation is only attempted if two normal hearts coexist in a single pericardium. Conjoined twins are classified according to the area of the bodies where the fusion takes place and the involvement of 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 they are called duplicata incompleta, and this category includes the most severe types of conjoined twins in which just few organs systems are duplicated. The most frequent varieties of conjoined twins are thoracopagus (40%), omphalopagus (33%), pygopagus (18%), ischiopagus (6%) and craniopagus (2%). Classification can be based on the theoretical site of union: 1) Ventral: · cephalopagus – fused from the top of the head down to the umbilicus. Two rudimentary (fused) faces, four arms and four legs. Lower abdomen and pelvis are separated. · thoracopagus – united face-to-face from the upper thorax down to the umbilicus, with heart involvement always. Four arms, four legs, two pelvises. · omphalopagus – joined face-to-face primarily in the area of the umbilicus, and sometimes involving the lower thorax, but always preserving two distinct hearts. There is not even a cardiac vessel in common. Two pelvises, four arms and four legs. · ischiopagus – united ventrally from the umbilicus to a large conjoined pelvis with two sacrums and two symphyses pubis. They appear more frequently joined end-to-end with the spine in a straight line, but they can also present face-to-face with a joined abdomen. Four arms, four legs, and in general, a single external genitalia and a single anus. 2) Lateral · parapagus – twins that share a conjoined pelvis, one symphysis pubis and one or two sacrums. When the union is limited to the abdomen and pelvis (does not involve the thorax) it is called dithoracic parapagus. If there is one trunk with two heads it is called dicephalic parapagus. If there is a single trunk and a single head with two faces they are diprosopic parapagus. Two, three or four arms, and two or three legs 3) Dorsal · craniopagus – united on any portion of the skull, except the face or foramen magnum. They share bones of the cranium, meninges, and occasionally brain surface. Two trunks, four arms and four legs · pygopagus – they share dorsally the sacrococcygeal, perineal regions and occasionally the spinal cord. There is one anus, two rectums, four arms and four legs. rachipagus- twins fused dorsally above the sacrum, involving different segments of the column. This type is extremely rare.
The diagnostic sign of conjoined twins is fusion of the bodies of monozygotic monoamniotic twins. Careful search for associated features and serial scans for confirmation are recommended to prevent misdiagnosis. Several sonographic signs may assist or prompt this diagnosis: · bifid appearance of the first-trimester fetal pole (‘V’ or ‘Y’- shaped twin pregnancy) · continuous skin contours at the same anatomic level · inability to separate fetal bodies · absence of an interamniotic membrane between the twins · more than three vessels in the umbilical cord · single heart · the heads and bodies of both twins are seen at the same level · unusual extension of the spines · unusual proximity of the extremities, even after external stimulation or maternal movement The presence of these signs varies according to the different types of conjoined twins. Conjoined twins must be considered whenever a monochorionic monoamniotic pregnancy is suspected. Discordant presentation does not exclude conjoined twins. Although the diagnosis of conjoined twins is easier during the first trimester, the type and severity of the condition is better achieved during the second trimester, when a more precise evaluation of the shared organs can be done. The diagnosis with 3D transvaginal sonography during the first trimester has also been described.
Conjoined twins have a unique presentation and the few differential diagnoses include lymphangioma, teratoma, or cystic hygroma.
Bifid appearance of the first-trimester fetal pole (‘V’ or ‘Y’- shaped twin pregnancy)
Continuous skin contours at the same anatomic level
Inability to separate fetal bodies
Permanent position of the fetuses relative one to another.