Conjoined Twins

Conjoined cephalo-thoracopagus twins


First trimester sonogram demonstrates conjoined twins with fusion of the thorax and part of the cranium.


Conjoined twinning was first described in 1100 AD and the first case diagnosed by ultrasound was reported in 1977. It is a rare abnormality estimated to occur in 1% of monozygotic twins and with an incidence between 1 in 50,000 to 1 in 100,000. Conjoined twinning have a female prevalence (75%-95%) and there is a higher incidence in non-Caucasions. It is thought to result from incomplete delayed division of the inner cell mass which occurs after the 14th day from fertilization. As differentiation of the chorion (placenta) and amnion have already occured, they are also monoamniotic and monochorionic. Thus idenitifcation of a dividing membrane or two placentas excludes the diagnosis.

Classification is based on symmetry and the size of the fused anatomic region. In symmetric forms, the name of the fused region is followed by the term pagus. If fusion is side by side and large, it is customary to name conjoined twins based upon the anatomic parts which remain unfused. Thus, dicephalus would mean two heads but fusion of the thorax and abdomen. The reported incidence of conjoined twins which are diplopagus (equally developed symmetric twins) are: thoracopagus (40%), xiphopagus or omphalopagus (34%), pygopagus (18%), ischiopagus (6%) and craniopagus (2%), the rarest form, occuring in one in 2.5 million births.

Types of Conjoined Twins

Inferior Conjunction Lower body is single or twins joined by some lower portion of body
Two faces with one head and body
Two heads with one body
Joined by inferior sacrum and coccyx
Joined by posterolateral sacrum and coccyx
Superior Conjunction Upper body is single or twins joined by some upper body portion
Single head, thorax, abdomen with two pelves and four legs
Facial fusion with or without thoracic fusion
Joined at the head
Middle Conjunction Fusion of the midportion of the body, separate above and below
Thoracic fusion
Omphalopagus (Xiphopagus)
Joined from umbilicus to xiphoid cartilage
Thoracic and abdominal fusion
Vertebral fusion above the sacrum
From: Barth RA, Filly RA, Goldberg JD, Moore P, Silverman NH. Conjoined twins: prenatal diagnosis and assessment of associated malformations. Radiology 177:201-207, 1990

There is a high incidence of congenital malformations in conjoined twins (10-20%) which are unrelated to the point of fusion. For example, anencephaly occurs 1.67 times more frequently in conjoined twins than in either dizygotic twins or singleton pregnancies. There is likewise a higher incidence of polyhydramnios (50-75%) than in normal twins (10%).

The sonographic and radiologic findings have been described by Gray et al and Koontz et al and Gore et al. These include 1) lack of a separating membrane 2) non-separable skin contour with an inablility to separate the fetal bodies 3) detection of other anomales in a twin gestation 4) solitary umbilical cord with more than 3 vessels present 5) both fetal heads persistently at the same level 6) backward flexion of the cervical spine (due to the fact that most conjoined twins are fused ventrally and face each other 7) bibreech or less commonly, bicephalic presentation and 8) constant relative fetal positions. It should be remembered however, that discordant presentation does not exlude the diagnosis, particularly in omphalopagus twins. The fused area may be quite small allowing for rotation of one of the twins.

The prognosis for conjoined twins in general is quite poor. 75% of conjoined twins are stillborn or die within 24 hours. Of thoracopagus conjoined twins, 75% have extensively joined hearts that preclude sucessful separation.


Barth RA, Filly RA, Goldberg JD, Moore P, Silverman NH. Conjoined twins: prenatal diagnosis and assessment of associated malformations. Radiology 177:201207, 1990

Chen HY, Hsieh FJ, Huang LH. Prenatal diagnosis of conjoined twins by real-time sonography: a case report. J Clin Ultrasound 11:94-96, 1983

Hubinont C, Pratola D, Rothschild E, Rodesch F and Schwers J. Dicephalus: unusual case of conjoined twins and its prepartum diagnosis. Am J Obstet Gynecol 149:693-694, 1984

Koontz WL, Herbert WNP, Seeds JW, Cetrulo CJ. Ultrasonography in the antenatal diagnosis of conjoined twins: a report of two cases. J Reprod Med 28:627, 1983

Gore RM, Filly RA, Parer JT. Sonographic antepartum diagnosis of conjoined twins. JAMA 247:3351-3353, 1982

Wilson RL, Shaub MS, Cetrulo CJ. The antepartum findings of conjoined twins. J Clin Ultrasound 5:35,1977

Abrams SL, Callen PW, Anderson RL, Stephens JD. Anencephaly with encephalocele in craniopagus twins: prenatal diagnosis by ultrasonography and computed tomography J Ultrasound Med 4:485-488, 1985

Gray CM, Nix HE, Wallace AJ. Thoracopagus twins: prenatal diagnosis. Radiology 54:398, 1950

Chatkupt S, Chatkupt S, Kohut G, Chervenak FA. Antepartum diagnosis of discordant anencephaly in dicephalic conjoined twins. J Clin Ultrasound 21:138-142, 1993

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