Abdomen

The midgut loop and its rotation

6 steps · 3 anomalies · Week 5 to Weeks 10 to 11

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Every question about a Meckel's diverticulum, an omphalocele or a volvulus in the newborn comes out of this one story. The midgut runs out of room, goes outside the body for a while, spins on the superior mesenteric artery, and comes back in a fixed order. If you can say which way it turns and what comes back first, the anomalies explain themselves.

Week 5

Step 1 of 6

What happens: The midgut forms a U-shaped loop suspended on a dorsal mesentery, with the superior mesenteric artery as its axis and the vitellointestinal duct at its apex.
What it forms: A cranial limb, which will become small intestine, and a caudal limb carrying the caecal bud.
What remains in the adult: The superior mesenteric artery remains the artery of the whole midgut.

Nothing in this set goes wrong at this step — it is here because the story does not make sense without it.

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Senior’s 30-second takeaway

The midgut loop and its rotation — the whole bench in four lines. Read this on the way in.

If you see this: A newborn with bilious vomiting · Painless rectal bleeding in a small child · An abdominal wall defect at the umbilicus
If asked to identify it: The midgut herniates, rotates 270 degrees anticlockwise about the superior mesenteric artery, and returns jejunum first.
If asked one more: The caecum comes back last and then descends into the right iliac fossa.
If asked why it matters: Incomplete rotation leaves a narrow mesenteric base that can volvulate around the superior mesenteric artery.

Then stop. Let the examiner ask the next question.

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Senior’s 30-second takeaway

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