The twins were fused at the lower halves of their bodies with a single umbilicus at the mid. There was a horizontal scar of about 3 ½ cm extending to the right of umbilicus. The healthier looking active pink neonate, termed Twin A, had normal head, neck and upper limbs. There was gradual broadening of the torso above umbilicus. The smaller neonate, termed Twin B, was a thin, emaciated, cyanosed, microcephalic neonate, with small torso and feeble reflexes. There were two separate lower limbs at the left side of Twin A and a fused lower limb at the right side. The anal orifice was absent and two orifices were found in perineum draining urine. Umbilicus had a single set of umbilical vessels. External genitalia were incompletely developed to assign gender to either neonate. Cardiac murmur was audible on auscultation in both the twins (Fig. ​(Fig.1,1, ​,22).

X-ray babygram revealed fusion at the lower abdomino-pelvic region. Scoliosis of dorsolumbar spine noted in both neonates, which was more marked in the Twin B. Pubic symphyses were widely separated and ischio-pubic bones were deformed on the right side with sacral dysgenesis of Twin B. The hip joints were abnormally oriented and horizontally oriented femora were attached to a large pelvic ring bilaterally. On one side, there was synostosis of femora. Calvarial defect with torticollis was present in Twin B (Fig. ​(Fig.33).

Ultrasound abdomen examination of Twin A showed hydronephrosis and hydroureter on both the sides while rest of the viscera were normal. In the Twin B, spleen and the left kidney were well formed but other viscera could not be clearly visualized. There was a large anechoic cystic structure in the pelvis representing bladder. Cystogram performed via the two small orifices in the perineum revealed non-visualization of urethra with pooling of contrast in the single reservoir representing shared urinary bladder. (Fig. ​(Fig.44)

MRI revealed moderate hydronephrosis bilaterally in both babies. In Twin B there was lack of visualization of other viscera. Normal lung and mediastinum were also not seen in Twin B who also had small skull, deformed and devoid of normal brain tissue, replaced by a cystic space. The Twin A showed normal thoraco-abdominal viscera bilaterally. Multiple prominent cystic areas were noted in the centre likely representing bowel loops. There was failure to identify separate urinary bladders. Because of the limited multi-dimensional views of MRI, internal genitalia were not appreciable. (Fig. ​(Fig.55)

Various classification systems have been proposed for this defect. Spencer classified conjoined twins on the basis of site of union i.e.: ventral or anterior union (cephalopagus, thoracopagus, omphalopagus, ischiopagus, and parapagus) and dorsal or posterior union (craniopagus, pyopagus, and rachipagus), while Potter and Craig simply classified on the basis of most common forms of twinning [6, 7].

Ischiopagus conjoined twinning considered the most complicated form, accounts only for 6% of the cases. Grossly, ischiopagus usually lie along a long axis with heads on opposite sides. They have a common umbilicus and the bodies fuse below this level, sharing lower abdomen and pelvis. About half of the ischiopagus have four separate lower limbs, 1/3rd have 3 lower limbs (2 separate and 1 fused) attached to the body laterally and 1/5th cases are parasitic [1, 2, 3, 4]. Our case, therefore, is an example of ischiopagus tripus variety. Radiologically, there was synostosis of the femora on the right side of Twin A. (Fig. ​(Fig.2).2). It is reported that one of the twins is almost always weaker or smaller, though genetically identical and may have additional congenital defects [5]. This was also noted in this case as the Twin B was very small, weak and microcephalic with scoliosis of dorsolumbar spine and sacral dysgenesis. Calvarial defect, torticollis and absence of normal brain tissue were also present.

The internal organs shared by ischiopagus conjoined twins are usually the liver, lower gastrointestinal tract (GIT) and genitor-urinary organs. The GIT sharing in ischiopagus twins is usually ileo-colic, having union at the terminal ileum (at Meckel’s diverticulum) with shared bowel distally or rarely the sharing is only colonic. The rectum and the anus may be single or imperforated. Obstructed GI or genitourinary tract, ruptured omphalocele, severe respiratory distress or cardiac failure in one of the twins, are some of the indications which may require emergency surgery [1, 2, 5, 8].

It is well known fact that the fetal wounds heal without scar formation. The true mechanism of the scarless fetal tissues repair is not known [9]. The linear healing wound or scar at the right side of umbilicus in our patient could not be explained according to this theory. This scar was also the line of vascular perfusion demarcation between the twins frequently noted when Twin B used to desaturate.

About 51% of the ischiopagus twins have shared pelvic organs. Urinary bladder may be single or double, lying side by side or combined with one bladder draining into the other [5]. In this case, the twins had two perineal openings which were draining urine with single urinary bladder as noted on cystogram, ultrasound abdomen and MRI.