History, classification, and two cases of conjoined twins

Fabrice Cuillier, MD*, K. Comalli Dillon, BA, RDMS**, Jean Michel Scemama, MD***, Thierry Gervais, MD***

*  Department of Gynecology, Felix Guyon Hospital, 97400 Saint- Denis, Ile de la Reunion, France. email: fabrice.cuillier@wanadoo.fr

**  Diagnostic Medical Sonographer, Editor, Translator, Novato, California. email: timbre440@yahoo.com

***  Radiologist, Chaussee street, Saint Paul 97460, Ile de la Reunion, France


Conjoined twins occur in approximately 0.2:10,000 births. The obstetrician and sonographer only rarely examine them. In Western countries, discovery of conjoined twins is often made by the end of the first trimester; however, in the third world, conjoined twins are often discovered only at birth. The authors report two cases: (1) discovery at 13 weeks gestational age of omphalopagus twins; and (2) birth of conjoined twins at 35 weeks gestation.

Clinical Cases

Case I

A 30-year-old patient, G4P3, with noncontributory medical history and no prior teratogen exposure, was seen in our clinic. A monochorionic, monoamniotic abdominal conjoined twin pregnancy (thoraco-omphalopagus; images 1, 2) was discovered on sonography at 13 weeks gestation. Four extremities were seen, as were two cardiac structures, two stomachs, and a liver (image 7). Amniocentesis revealed 46, XY; the patient chose termination of pregnancy.

Postmortem analysis verified a common subumbilical abdominal wall with peritoneum and liver crossing as a bridge behind it. The umbilical cord had four vessels on the fetal side of the placenta: a vein and an umbilical artery for each fetus in the shape of a ‚Äúy‚ÄĚ in the cord interior.¬†One vein and one artery were seen on the maternal¬† side of the placenta.¬†Part of the common colon formed a median sac of 1 cm in diameter with meconium in the proximal and distal regions of the digestive tube of each fetus.The rest of the viscera were unique to each twin; however, there was a defect of the superior sterno-abdominal wall in each, revealing ectopia cordis with truncus arteriosus type I in each fetus.¬†The placenta was monochorionic and monoamniotic.¬†We concluded that we saw omphalopagus twins with a shared umbilical cord.

Image 1: Sagittal section showing thoraco-omphalopagus twins
Image 2: Sagittal section showing thoraco-omphalopagus twins
Image 3: Color Doppler sagittal image of thoraco-omphalopagus twins
Image 4: twins and transverse plane through thoraces
Image 5: Transverse section through thoraces and pelves.
Image 6: Transverse section through thoraces and pelves.
Image 7. Transverse section showing two stomachs and common liver.

Case II

While we served on a humanitarian mission to Gujarat, India, a patient came to the emergency department of Bharush Hospital at around 35 weeks gestation. She presented with violent uterine contractions caused by polyhydramnios. Sonography performed with outdated equipment confirmed polyhydramnios and a fetus lodged in the posterior uterus, seeming to consist of a sole cephalic pole and four extremities (cephalothoracopagus). Unfortunately, images could not be preserved. The fetus was delivered vaginally without dystocia and proved to be conjoined twins with no cardiac activity (image 8).  Autopsy was declined by the mother.

Image 8: Cephalothoracopagus twins.


Only four cases of conjoined twins were seen before birth radiographically, prior to the early sonographic era of the 1960s. Otherwise discovery was made at birth, as in third-world countries and in our second case.  In 1967, Rudolph reported prenatal radiographic discovery of 16 out of 65 cases of conjoined twins by radiography [7]. In 1976, Wilson made a radiographic discovery at 35 weeks of conjoined twins at 35 weeks [12]. In 1989, Grutter observed thoracopagus twins at 16 weeks [8]. Sonography has demonstrated conjoined twins at the end of the first trimester since 1995 [1]. In 1995, Maggio reported a diagnosis of conjoined twins (thoracopagus with a single heart) at 8 weeks, 5 days, confirmed at 13 weeks gestational age [14]. In 1997, Hill diagnosed conjoined twins at 6 weeks, 6 days, confirmed at 10 weeks gestation. More precise analysis of Hill’s case was made at 11 weeks, 6 days revealing ischiopagus twins with omphalocele and ectopia cordis. Sheng Yin recently diagnosed conjoined twins by embryoscopy. Maymon reported 3D sonographic analysis of conjoined twins at 10 weeks GA; he prefers 2D to 3D sonography for diagnosis [16]. Fast MRI is becoming an important tool for prognostic assessment, in the first trimester for anatomic evaluation, and later in pregnancy to plan the route of delivery and separation surgery when possible. Serial sonography at 12-13 weeks of gestational age confirms common anatomy and is recommended to prevent false-positive diagnosis [17-18]. Early sonography shows continuity between the fetuses and fails to show independent movement. No amniotic membrane will be seen between the twins.

First-trimester endovaginal sonographic indications for possible conjoined twins include:

  • A sole extra-amniotic vitelline vesicle is remarkable for conjoined twins¬† since the vesicle is usually enclosed between¬†amnion and the chorion
  • A sole embryo with a bifid appearance seen before 10 weeks GA
  • Probe motion of the probe which moves both embryos at the same time
  • Visualization of two hearts or two stomachs


Conjoined twins occur in approximately 0.2:10,000 pregnancies [2], but only 0.05:10,000 live births. There is a greater incidence (70%) of female conjoined twins [3-4], but the reason is unknown.  Neither conjoined triplets nor recurrence of conjoined twins [8] has been described [7]. There is no mention in the literature of conjoined twins born to conjoined-twin parents.


Aristotle (384-322 B.C.) wrote about conjoined twins in his memoirs [1].¬†The term ‚ÄúJaniceps,‚ÄĚ indicating a form of conjoined twins, comes from the name Janus, a Roman god with two faces (Roman era, approximately 170 AD) [2].¬†Conjoined twins called the ‚ÄúBiddenden Twins‚ÄĚ survived for 30 years in Kent, England, around 1100 AD.¬†Antoine Par√©, in the 17th century, gives an account of craniopagus twins who lived until 10 years of age.¬†Overall, approximately 500 cases of conjoined twins are reported in the literature [3, 4].¬†A system of classification for conjoined twins, established by St-Hilaire in 1832 is used today [5].


Monoamniotic twin pregnancies comprise 1% of monozygotic twin pregnancies (approximately 1:25,000 pregnancies) [6]. Placentation in twin pregnancies depends on the stage of embryonic division [3]. If cell differentiation takes place after the 14th day of embryonic development, it is incomplete, resulting in conjoined twins.


The etiology of conjoined twins is not known. In general there are no karyotypic abnormalities [5], nor do race, heredity, birth order, or consanguinity appear to influence the process. Two prominent theories explain the appearance of conjoined twins: the theory of fusion and the theory of fission. In the fusion theory, authors propose that the twins join secondary to late embryonic division. In the fission theory, some authors feel there is an absence of complete embryonic division.


The prognosis for conjoined twins is generally unfavorable, with approximately 40% of cases stillborn [7]. The worst prognoses concern craniopagus twins and those with a sole cardiac mass [11]. Structural anomalies are frequently found such as polyhydramnios (50%), cardiac malformations, common omphaloceles [18], and neural tube defects. Upon discovery of nonviable conjoined twins, interruption of pregnancy should therefore be recommended [7].


In the case of potentially viable conjoined twins, after 24 weeks GA the choice between vaginal delivery or prophylactic caesarian section should be made based on maternal safety and neonatal criteria. Caesarian section avoids dystocia, uterine rupture, and fetal death in utero [21]. Approximately six to ten cases of conjoined twins per annum worldwide are treated surgically. The surgery is most successful when commonality of fetal organs is limited [22]; surgical intervention often takes place around one year of age.

Types of classification

Conjoined twin classification establishes the function and importance of the site of union, common organs, and their symmetry/asymmetry. Duhamel bases classification on internal anatomic relationships so as to evaluate prognosis [3]. Wilder classifies conjoined twins by external attachment, permitting distinction of autosite (independent) twins from parabiotic twins (one normal and one parabiotic, e.g. fetus in fetu, usually discovered at birth) [9]. Pathogenesis of this entity is obscure. It seems to originate from a heteropagus twin inside the second twin, probably the result of a monochorionic diamniotic twin pregnancy with anastomosis of vitelline circulation [9].

Common structures between conjoined twins vary. Classification can be based on fused structures, as Dollander did, using the classification system of St. Hilaire.¬†The Greek suffix -pagus, meaning ‚Äúwhat is fixed,‚ÄĚ differentiates type:

St-Hilaire’s classification [8]
Craniopagus¬†twins are classified as ‚ÄúTerata anadidyma‚ÄĚ (proximal fusion)
Thoracopagus and omphalopagus are classified as “Terata anacatadyma “ (median fusion)
Ischiopagus and pygopagus¬†twins are classified as ‚ÄúTerata catadyma‚ÄĚ (distal fusion)

Conjoined twins joined in the form of an H
Craniopagus: Cephalic fusion, with parietal fusion seen most often
Thoracopagus or sternopagus: Fusion of the upper thorax: 100% of cases have hepatic cases and 75% have cardiac fusion [7]
Omphalopagus: Having common abdomens
Ischiopagus: Anterior union of the lower body
Pygopagus: Union of coccyx and sacrum

Teratodelphic twins, united high on the body in the form of a lambda (őĽ)
Janiceps or cephalopagus: Anterior union of the superior body with two faces pointing away from each other on a fused head [10]
Thoracodelphic or iniote: Union of faces and thoraces
Pelvidelphic: The head is attached to the pelvis

Teratodymic twins united caudally in the form of a Y
Iniodymic: Joined at the occiput
Xyphodymic or sternodymic: Joined at the sternum
Thoracodymic Joined at the thorax

further type of classification is by axis. Conjoined twins with parallel axis (paropagus) are side-to-side on a parallel axis. Conjoined twins with opposite axis (teratopagus, crucipagus) have face-to-face union. Both parallel (paropagus) and crossed-axis  (crucipagus) twins are fused at the trunk. They are classified in the following ways:

Conjoined twins with parallel axis, or paropagus
Cephalic parapagus twins Cephalic parapagus twins have separate heads side by side
Mesophagus¬†twins Conjoined twins in an ‚ÄúX‚ÄĚ shape due to median fusion
Caudal parapagus twins Union at lower extremities and pelvis

Twins conjoined on an opposite axis, or crucipagus twins
Cephalic crucipagus twins, each with its own cranium
Caudal crucipagus twins

Anatomic types of conjoined twins (adapted after Lutz) [25]

Teratopagus twins

a) Cephalopagus
b) Thoracopagus
c) Pygopagus

Teratodymic twins

d) Iniodymic
e) Sternodymic
f ) Thoracodymic

Teratodelphic twins

g) Pelvidelphic
h) Thoracodelphic

The umbilical cord also has classifications; if it is single, it is ‚Äúmonomphalian;‚Ä̬† if double, ‚Äúeusomphalian.‚ÄĚ

The conjoined twins in Clinical Case I were:

  • isodiploteratat and autosite (union of two equal twins);
  • monomphalian (with¬† sole umbilical cord); and
  • Terata anacatadidyma (omphalopagus with midline fusion).

The twins in Clinical Case II were:

  • isodiploterat¬† and autosite (union of two equal twins);
  • monomphalian, (with¬† sole umbilical cord); and
  • Terata anadidyma (omcraniopagus with proximal fusion).


Diagnosis of conjoined twins is possible as early as 8 weeks gestation, but accurate evaluation of common structures is not possible. The 12-week scan allows clinicians to assess viability, and, in the case of nonviability, to propose early medical interruption of pregnancy, preventing hysterotomy in the case of a delayed termination. Early discovery of viable conjoined twins permits assessment of the best route of delivery and a planning for serial sonography and fast MRI to plan separation surgery.


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