According to Dr. Lee, the quickest option is to quickly check your birth certificate because blood type is occasionally stated in birth records. However, this information isn't always accurate, so it's best to verify with your doctor or hospital. The American Red Cross also advises that blood types are not usually known until about a year after birth because antibodies can change their appearance. Therefore, if you cannot find any documentation of your blood type, try to remember what factors might have caused you to become allergic to blood. For example, if you had many surgeries as an infant, you may have lost some blood during these procedures and your body would have made antibodies against the blood proteins. These antibodies could then show up in a blood test later in life. If you cannot find anything related to your blood type on your birth certificate or medical record, there are other ways you can determine yours. The American Red Cross says that most people know their own blood type, but some people may not realize it.