What happens to a hemangiomas without treatment?

Most Do Well. Infantile hemangiomas have a growth phase and an involutional phase. The growth phase occurs over the first 4-10 months of life, with few having significant growth after 5 months. All hemangiomas involute slowly over 2-10 years, but not all involute completely-- possibly leaving irregular texture, loose tissue and can scar if they ulcerate. Treatment may be necessary in large or growing lesions.
Usually nothing. Most hemangiomas are an incidental finding and do not cause problems. Usually they remain stable in size and do not grow. It depends on where they may be located, as some may grow or be in a sensitive location. Some rarely will grow large enough to trap platelets. Most of the time a hemangioma can be left alone without consequences.