The midgut loop of the developing embryo elongates by herniating out into the umbilical cord from weeks six to fourteen of intrauterine life. The apex of this loop communicates with the yolk sac by the vitellointestinal duct - the site of a possible Meckel's diverticulum.
The loop of gut makes a counter clockwise rotation of 270 degrees around the superior mesenteric artery whilst in the extra-embryonic coelom. The gut starts to return to the abdominal cavity at about 14 weeks. In the initial descent of the gut back into the abdomen the proximal jejunum lies in the left upper abdomen.
The last part of the gut to re-enter the abdomen is the caecum and this initially lies in the subhepatic position. The process of rotation and descent is complete when the gut aligns itself in adult position with the caecum in the right iliac fossa.
Failure of the gut to return to the abdomen results in omphalocoele.
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