Some doctors are not board certified because their particular specialty is not recognized by ABMS, some are not board certified because they began practicing before ABMS existed, some have difficulty passing the tests (which doesn't necessarily mean that they are bad physicians), and some are recent graduates who have not had time to certify. The people in the last category are called Board Eligible, because some specialties require that you practice for a minimum amount of time before you are allowed to be tested (usually less than 2 years). If you want to know if your doctor is Board Certified or Board Eligible, check the website at ABMS.org. Click on the link that says "Is my doctor certified?” You will have to register, but there is no cost to use the service.
Medicine is constantly changing. There are things that you learn in medical school that are later found to be absolutely false! Once you leave the academic arena of medical school, the only way to stay up to date on what is going on in medicine is to be in a state of constant learning (hopefully learning the correct things, because you no longer have a teacher to guide you). Board certification forces you to do just that. In order to remain board certified, doctors are required to re-certify every 6 years in OB/GYN (other specialties differ). Take control of your health by selecting physicians who care about life long learning and staying current in the areas that matter most to you!