Wilms tumors can be hard to find early because they can grow quite large without causing any pain to the child. Children may look healthy and play normally. The first sign is usually swelling in the abdomen. Chances are parents will notice the swelling or hard mass in the child's belly while bathing or dressing the child. It will feel firm and probably be large enough to be felt on either side of the abdomen. It is usually not particularly tender or painful. About 25% of children with Wilms tumor may also have stomach pain, fever, blood in the urine. If your child has these signs and symptoms, see a doctor right away. Another sign of this cancer is high blood pressure that may show up in the doctor's exam.