physiologic umbilical herniation

**Embryology/timeline** - Week 6: The midgut elongates faster than the abdominal cavity grows (also limited by the large liver and mesonephric kidneys), so the midgut loop herniates into the umbilical cord (extraembryonic coelom). - The midgut loop is supplied by the superior mesenteric artery (SMA), with the vitelline (omphalomesenteric) duct at the apex of the loop. The cranial limb becomes distal duodenum, jejunum, and proximal ileum. The caudal limb becomes distal ileum, cecum, appendix, ascending colon, and proximal 2/3 of the transverse colon. - While herniated, the loop rotates 90° counterclockwise (viewed from the front) around the SMA axis. - Weeks 10–11: The loop returns to the abdominal cavity and undergoes an additional 180° counterclockwise rotation, for a total of 270°. The jejunum returns first, and the cecum returns last, ending up in the right lower quadrant. - The gut is fixed in place: the ascending and descending colon become secondarily retroperitoneal, and the duodenum (except the first part) becomes retroperitoneal.

**Clinical correlations (high-yield)** - **Omphalocele:** The gut fails to return to the abdomen. Abdominal contents herniate through the umbilical ring and are covered by a sac of peritoneum and amnion (membrane-covered). The umbilical cord inserts onto the sac. It is associated with other anomalies, including trisomies 13 and 18, Beckwith–Wiedemann syndrome, and cardiac and neural tube defects. Elevated maternal serum AFP is possible. - **Gastroschisis:** This is not a failure of physiologic herniation. It is a paraumbilical full-thickness abdominal wall defect, usually right of the umbilicus, from a lateral body fold closure defect or vascular compromise. There is no covering sac, and the bowel is exposed to amniotic fluid. It has fewer associated anomalies and elevated AFP. Risk factors include young maternal age, smoking, and drug use. - **Malrotation/midgut volvulus:** Abnormal rotation or fixation can cause midgut volvulus around the SMA. This presents in infancy with bilious vomiting. A "bird's beak" appearance or a corkscrew duodenum is seen on UGI series, and the SMV lies to the left of the SMA. Ladd bands can obstruct the duodenum. This is a surgical emergency. - **Meckel diverticulum:** A persistent vitelline duct remnant, following the "rule of 2s" (2% of the population, 2 feet from the ileocecal valve, 2 inches long, 2× more common in males, presents by age 2). It may contain ectopic gastric or pancreatic tissue (Tc-99m pertechnetate scan) and can cause painless rectal bleeding. - **Umbilical hernia vs physiologic herniation:** A true umbilical hernia is a postnatal fascial defect at the umbilical ring. It is common in Black infants and in premature infants, and it usually closes spontaneously by age 3–5 years.

**Pearls** - Physiologic herniation is normal before week 10 and pathologic after that. An ultrasound seeing bowel in the cord at 8–9 weeks is normal. - Mnemonic: 6 weeks out, 10 weeks back in, and 270° counterclockwise total. - Omphalocele = covered sac, associated anomalies. Gastroschisis = no sac, usually isolated, and intestinal atresia is a common complication.