Box for secondary retroperitoneal organs: These organs are intraperitoneal early in development. Later, their mesenteries are pushed back and fuse with the posterior abdominal wall, so the organs become retroperitoneal. These organs are the pancreas, duodenum, ascending colon, and descending colon.
Embryonic remnants
| Embryonic origin | Remnant |
| Foramen ovale | Fossa ovalis |
| Ductus arteriosus | Ligamentum arteriosum |
| Ductus venosus | Ligamentum venosum |
| Umbilical vein | Ligamentum teres hepatis |
| Umbilical arteries | Medial umbilical ligament, part of internal iliac and superior vesical arteries. |
| Mesonephric or Wolffian ducts | In males, it becomes the efferent duct system, including the epididymis, vas deferens, and seminal vesicles. In females, it persists as the epoophoron and paroophoron, and as Gartner’s ducts. |
| Paramesonephric or Mullerian ducts | In females, it becomes the fallopian tubes, uterus, cervix, and upper part of the vagina. In males, remnants are seen in the prostatic utricle. |
| Urachus or allantoic stalk | Median umbilical ligament |
| Notochord | Nucleus pulposus |
Meconium ileus presents with bilious vomiting and abdominal distension, with a high risk of perforation and peritonitis.
Prenatal ultrasound shows hyperechoic masses from thick meconium, dilated bowel loops, and non-visualization of the gallbladder.
Postnatal imaging shows dilated bowel loops with or without air-fluid levels (often absent because the meconium is dry), a “soap bubble” or mottled appearance, and in some cases abdominal calcifications. The distal colon shows a microcolon appearance due to disuse.