Conjoined Twins

Conjoined twins are defined when two twins are fused together by the same body part in a monochorionic monoamniotic twin pregnancy.  

LD2G 

Conjoined twins represent a rare subgroup of monochorionic monoamniotic pregnancies, with an estimated incidence of 1.5 per 100.000 births (1), complicating approximately 1% of monochorionic pregnancies (2). As in monoamniotic twins in general, female fetuses are more frequently affected by this condition, with a 3:1 female-male ratio (3).  

The precise embryology of conjoined twins remains uncertain. All monoamniotic pregnancies usually result from a single fertilized egg; some authors propose that conjoined twins result from later embryonic splitting compared to other monoamniotic pregnancies (usually 12-15 days from conception) (1,3-4). Other authors attribute the possible cause of this phenomenon to be the secondary fusion of the two products of the conception (3). Finally, some have described the presence of biovular follicles in the female ovaries, leading to two conjoined gametes that will be fertilized (4). 

Conjoined fetuses can be classified into different types according to the site of the body fusion to the co-twin (1,3-5): 

• Thoracopagus (42%): is the most common type of conjoined anomaly; the fetuses are connected by the thorax, up to the umbilicus. This variety is usually characterized by the fusion of the abdomen and the sharing of the liver. The morbidity and mortality are very high due to cardiovascular anomalies associated (e.g. one single conjoined heart or two fused hearts).   

• Parapagus dicephalus (11.6%): the site of union includes the lower abdomen and the pelvis; they usually share the same thorax. The cranium might be fused or separated; anencephaly is often observed. They usually share a liver, lower gastrointestinal tract, gallbladder, and pancreas.  

• Cephalopagus (5.5%): these twins are fused from the head to the umbilicus, they present with 2 faces on the opposite sides of the head. The upper gastrointestinal (GI) tract is shared. They usually have two different central nervous systems and two cardiac pumps.  

• Omphalopagus (5.5%): fusion occurs between the sternum and the umbilicus; the fetuses usually share the liver and the GI tract.  

• Ischiopagus (1.8%): twins share the lower tract of the abdomen and the pelvis, like in the parapagus variant, but in this case the fusion is end-to-end, with shared genitalia and lower GI tract.  

• Craniopagus (3.4%): the fusion site is the cranium, but it never involves the skull base or the foramen magnum; therefore, the two twins might be reciprocally oriented in any position, with two different faces. They usually present with fused meninges, but two distinct brains. The rest of the body appears normal.  

• Rachipagus (1%): the vertebral columns and the spinal cords are fused. The limbs and the rest of the anatomy usually appear normal.  

• Pygopagus (1%): the fusion involves the sacrum and the perineum; the lower tract of the spinal cords and dura are conjoined. The fetuses share the same pelvis, genitalia and might have limbs anomalies.  

• Unspecified (21.4%). 

The first five variants are called “ventral” variants; the others are defined “dorsal” variants. 

Monochorionic monoamniotic pregnancies are frequently complicated by congenital anomalies. Conjoined twins often present with severe cardiovascular or cerebral conditions, depending on the site of fusion, but they also carry a higher risk of developing associated anomalies, such as gastrointestinal abnormalities (Meckel’s diverticulum, bowel atresia, anomalous hepatic venous drainage), genito-urinary anomalies (duplex kidney, renal dysplasia, pelvic-ureteric junction obstruction, vescico-ureteric junction obstruction), musculoskeletal anomalies (congenital dislocation of the hip, talipes) (4).  

Evidence about the risk of recurrence of conjoined twins is scarce; however, since no association with maternal age, race, parity, or heredity has been observed, risk of recurrence is considered negligible (6). 

Diagnosis is usually made with prenatal ultrasound. Conjoined twins are generally suspected at early first-trimester ultrasound, when chorionicity and amnionicity are assessed. They present as embryonic or fetal poles that appear very close to each other and do not change position throughout the duration of the scan. The pregnancy must be defined as monochorionic monoamniotic, in the absence of two distinct placentas and intertwin membrane (2).  

Later in pregnancy, if not diagnosed before, other findings might suggest this diagnosis, such as inseparable fetal parts, no separated movements of twins, less limbs than expected hyperextension of cervical tract of the spine, same fetal presentation, apparently single cord with more than 3 vessels (4-8). A detailed examination of the shared anatomy is of great importance in cases of conjoined twins to allow proper identification of the type of condition. In this context, the use of Color Doppler (CD) may be useful to characterize the shared organs: the application of CD on the head structures of suspected craniopagus twins may identify some shared vascular structure, attributable to the sagittal sinuses. (9) Similarly, the application of CD in the abdominal area of a suspected case of thoracopagus, parapagus or omphalopagus, may identify a large vessel below the liver area with arterial waveform at pulsed Doppler, or two vessels with opposite blood flows, indicative of shared liver. The umbilical vein may be characterized as entering the shared liver as a unique vessel or two different umbilical veins may join two parts of the shared organ. (9) 

In addition, increased nuchal translucency, cystic hygroma or polyhydramnios might be observed.  (10) 

Cojoined twins must be distinguished from uncomplicated monochorionic monoamniotic pregnancy, where the positions of the fetuses or the presence of cord entanglement limiting movement of the fetus relative to each other might mimic the situation observed in conjoined twins.  

The final diagnosis is made when the fusion of fetal parts is evident. The anatomy scan is useful to extensively describe the location and extent of the fused area; 3D ultrasound might help visualise the relationship between the body parts. A detailed fetal echocardiography is recommended for detection of additional cardiac anomalies. (3) 

Fetal MRI can be useful in unclear cases, or to better assess the tissues’ involvement, to achieve a detailed plan for post-natal surgery. (4-8) 

Conjoined twins carry a very high risk of morbidity and mortality, with up to 60% of cases of fetal demise. A review of 379 cases reported a live birth rate of 45%, with 27% resulting in stillbirths and 27% in terminations of the pregnancy. (7) 

The prognosis of the liveborns mainly depends on the site of fusion, on the organs that are shared and the presence of associated anomalies, in particular cardiovascular anomalies, that influence the surgical separability.  

The best chance of survival is seen in omphalopagus twins, because of high surgical intervention feasibility. Cephalopagus and thoracopagus are usually not viable and surgery is not performed. All other types of conjoined twins need extensive evaluation to assess operability. (4) 

Even when surgery is feasible, the postoperative mortality rate remains high, mainly due to infections or complications related to associated anomalies. (4) 

Multidisciplinary assessment and counselling are mandatory in the presence of conjoined twins. These cases should be always referred to tertiary centers with high expertise in fetal medicine, neonatal surgery, and radiology. (2) The option of termination of pregnancy is offered to parents affected by conjoined twins.   

For ongoing pregnancies, the mode of delivery indicated in these cases is cesarean section (2,10), because of very high risks of obstructed labor, shoulder dystocia and uterine rupture in case of vaginal delivery. Evidence regarding the optimal timing of delivery in conjoined twins is very scarce, but 35 weeks of gestation has been defined as a reasonable timing due to the high risk of stillbirth (10).  

1. Mutchinick OM, Luna-Muñoz L, Amar E, et al. Conjoined twins: a worldwide collaborative epidemiological study of the International Clearinghouse for Birth Defects Surveillance and Research. Am J Med Genet C Semin Med Genet. 2011;157C(4):274-287.  

2. Khalil A, Rodgers M, Baschat A, Bhide A, Gratacos E, Hecher K, Kilby MD, Lewi L, Nicolaides KH, Oepkes D, Raine-Fenning N, Reed K, Salomon LJ, Sotiriadis A, Thilaganathan B, Ville Y. ISUOG Practice Guidelines: role of ultrasound in twin pregnancy. Ultrasound Obstet Gynecol 2016; 47: 247 – 263.’  

3. Spitz L. Conjoined twins. Prenat Diagn. 2005;25(9):814-819.  

4. Bindlish A, Sawal A. A Detailed Description and Discussion on Conjoined Twins. Cureus. 2022;14(9):e29526. Published 2022 Sep 24.  

5. Baken L, Rousian M, Kompanje EJ, et al. Diagnostic techniques and criteria for first-trimester conjoined twin documentation: A review of the literature illustrated by three recent cases. Obstet Gynecol Surv 2013; 68:743.  

6. Melo Â, Dinis R, Portugal A, Sousa AI, Cerveira I. Early prenatal diagnosis of parapagus conjoined twins. Clin Pract. 2018;8(2):1039. Published 2018 Mar 28.  

7. Mahalingam S, Dighe M. Imaging concerns unique to twin pregnancy. Curr Probl Diagn Radiol. 2014;43(6):317-330.  

8. Winkler N, Kennedy A, Byrne J, Woodward P. The imaging spectrum of conjoined twins. Ultrasound Q. 2008;24(4):249-255.  

9. Mehollin-Ray AR. Prenatal and postnatal radiologic evaluation of conjoined twins. Semin Perinatol. 2018 Oct;42(6):369-380.  

10. O'Brien P, Nugent M, Khalil A. Prenatal diagnosis and obstetric management. Semin Pediatr Surg. 2015;24(5):203-206.  

Please note that the maximum upload size is 5MB, and larger images and video clips can be sent to [email protected].