Hi and welcome to HCM Unexplained bruising once in while is understandable but if occurs frequently should be investigated thoroughly since it can indicate an underlying medical condition or ailment. it has been observed that bruises are often observed more on women than men. Women often complain of bruising on legs, thighs, buttocks and upper arms. Thinner people are more prone to bruising as they lack the fat cushion that helps prevent injury. It may also be due to deficiency of vitamin C, vitamin B12, folic acid and vitamin K. Many times taking medications like aspirin, ibuprofen, non steroidal anti-inflammatory drugs, and even birth control pills increases chances of sudden bruising. Topical and systemic corticosteroids cause skin thinning, making one prone to bruises. Many a time bruising can be caused due to liver ailments, kidney diseases and blood disorder. I do hope that you have found something helpful and informative and I will be glad to answer any further query. Take care.