Many blood vessels must be cut in order to divide the stomach and to move the bowel. Any of these may later begin bleeding, either into the abdomen (intra-abdominal hemorrhage), or into the bowel itself (gastrointestinal hemorrhage).
Transfusions may be needed, and re-operation is sometimes necessary. Use of blood thinners, to prevent venous thromboembolic disease, may actually increase the risk of hemorrhage slightly.