Physiological umbilical hernia

The liver and the midgut grow so quickly that soon there is not enough space in the abdominal cavity. As the passage between the intra- and extra-embryonic coelom (umbilical coelom) is still open, it is possible for a rapidly-formed loop of the intestine to leave the abdominal cavity through here and enter the umbilicus. A physiological umbilical hernia can be formed which is present from the end of week 5 (10 mm, S16) until week 9 (40 mm). This umbilical hernia is deemed physiological because it appears within normal development. During the same period of development, the lesser omentum, greater omentum and lesser sac are formed.
The umbilical loop is initially in a sagittal plane. As it enters the umbilicus (umbilical coelom), it becomes rotated through approximately 90 degrees in a clockwise direction (as seen from the perspective of the embryo). The rostral (proximal) limb thus then ends up on the right hand side, and the caudal (distal) limb on the left (S16). The two limbs of the loop are now in a horizontal plane. The rostral limb grows much more quickly than the caudal limb and forms a series of loops in the umbilical coelom. (Animation: Intestinal rotation)
The cause for the return of the intestine to the abdominal cavity is not known. It is possible that more space is created in the abdominal cavity because the embryo straightens up during this period. The loops of the rostral limb are the first to re-enter the abdominal cavity, ending up in the left inguinal region. This results in an additional rotation of 90 degrees. The initially caudal branch folds back towards the right, on top of the intestinal loops, where it occupies the higher and more ventral part of the abdominal cavity. The caecum and the vermiform appendix are located in the upper right quadrant of the abdominal cavity (sub-hepatic caecum). (Animation: Intestinal rotation)
The colon, caecum and mesentery change position throughout the fetal period. Their final position is reached only in the early postnatal period. This development comes about mainly because of the displacement of the caecum in the iliac fossa. (Animation: Intestinal rotation)