Bowel obstruction usually occurs after 20WG.
The gestational age at onset onset of dilatation is actually not reliable enough to predict the level of obstruction between proximal or distal small bowel.
However, large bowel obtructions indeed tend to reveal themselves at later stages (although the dilation diameter alone in large bowel is far from being predictive of a pathologic condition alone).

The common underlying causes of small bowel prenatal obstruction are :
-single or multiple atresia(s), which may be associated with segmental volvulus.
-meconial ileus, in a fetus with Cystic Fibrosis
-complicated bowel duplication
-complicated bowel malrotation
-apple-peel syndrome (small bowel fed by archaid mesenteric arteries, rotating around a small meso in an "apple-peel" fashion)

The accuracy of US (2B and Doppler) alone is far from being conclusive on the underlying cause. Fetal abdominal MRI accuracy is still being investigated on several practical points : level of obstacle, prediction of multiple atresias, length of the remaining intact bowel, associated volvulus, associated malrotation, meconial ileus pattern. On the other hand, fetal abdominal MRI normal landmarks have been very well established according to gestational age.

We report a case of proximal small bowel obstacle with liquid-like/non meconium-like content signal and non visible colon. Upon neonatal surgery, the surgeon indeed found one proximal jejunal atresia, followed by seven small bowel atresias. Fortunately, the length of the remaining bowel was long enough (>1meter) not to induce short bowel syndrome.