We report the rare case of a pregnant woman who had undergone Roux-en-Y gastric bypass 8 months previously, and now presented with subacute small bowel obstruction secondary to internal herniation of some of the proximal Roux limb into the lesser sac through the transverse mesocolon rent, which was widely spread apart. At laparoscopy, the hernia contents were reduced and the defect was repaired. The patient made a good recovery. Because of the changes associated with pregnancy, gastric bypass patients may be at an increased risk of internal herniation. It is particularly important not to delay surgical exploration, even in the absence of a positive finding on imaging, because delay may lead to potentially devastating bowel strangulation and sepsis culminating in loss of fetus and mother.