Low platelets, also called low platelet counts, occur when levels of blood cells (called platelets) are lower than normal. Platelets help the blood to clot, so low platelets can cause excessive bleeding because the blood can’t clot normally. Low platelets is also known as the condition thrombocytopenia. Low platelets commonly occur as a side effect of other health conditions, including immune system problems and autoimmune diseases, pregnancy, or leukemia. Low platelets can also occur as a result of medication side effects. Low platelets can cause you to bruise easily and frequently, bleeding that won’t stop, blood in the urine, and blood in the stools.