Have you seen a doctor? Here's a partial list of things known to cause them. If they continue more than a couple days, you'd do well to see a doctor. Usually they're just a harmless spasm of the diaphragm but sometimes there's a more serious cause behind it.
Some illnesses for which continuing hiccups may be a symptom include: pleurisy of the diaphragm, pneumonia, uremia, alcoholism, disorders of the stomach or esophagus, and bowel diseases. Hiccups may also be associated with pancreatitis, pregnancy, bladder irritation, liver cancer or hepatitis.
