A condition for which medical advice was given or treatment was recommended by or received from a physician within six months before the effective date of coverage is considered a pre-existing condition. Prior to purchasing a Medigap policy, one should know if the pre-existing condition will be covered by the policy. Some Medicare Supplements cover pre-existing conditions immediately. Some do not. An insurance company can accept an applicant and not cover the condition for up to no more than 6 months. This a "pre-existing condition waiting period". For this to be applied the condition must be either diagnosed or treated during the 6 months prior to the Medicare Supplement starting. Medicare will still cover the condition, but the insurance policy will not.
If the insured is 65 years of age or older and has had a continuous period of prior creditable coverage for at least 6 months, pre-existing conditions will not be excluded. If the insured has had a continuous period of creditable coverage for less than 6 months, pre-existing conditions will be excluded by the aggregate of the period of creditable coverage applicable to the applicant as of the enrollment date. This means if you have been on an individual major medical policy, a group major medical policy, or Medicare Part A for example the insurance company will go ahead and waive the waiting period.