Gangopadhyay et al. Journal of Medical Case Reports 2010, 4:96 http://www.jmedicalcasereports.com/content/4/1/96 Page 2 of 7 Figure 1 Plain X-ray of the vertical calcification on the left side of the abdomen. that had a bony outline resembling a fetus (Figures 2 and 3). Interestingly, we found nothing significant in the baby’s family history. We performed an elective laparotomy after correcting the baby’s fluid and electrolyte levels. We then found a well-encapsulated cystic retroperitoneal mass that was displacing his spleen, transverse colon and pancreas. This displacement presented laterally and caudally toward his cephaloid and left kidney (Figure 4). The mass had a separate blood supply connected to the baby’s abdominal aorta just below his left renal artery. We mobilized, without complication, his left colon, pan- creas, duodenum and small bowel, after which we were able to excise the mass completely. The sac contained two miniature fetuses connected to each other by a cord-like structure at the umbilicus. The miniature fetuses had a well-defined foot, skin with hairs, a convex and pliable skull bone, and other undifferen- tiated tissues (Figure 5). A radiograph of the specimen showed cranial bones and long bones with vertebral col- umns (Figure 6). We then performed a macroscopic pathological examination, from which we were able to note that the mass measured 20 × 8 × 5 cm. It was also composed of a head with hair, a trunk, and rudimentary limbs connected by cord-like structures. The mass corre- sponded to an incompletely developed twin fetus. A microscopic examination showed that the underde- veloped twin had mature embryonic tissues containing