Your initial enrollment window allows you to enroll in all four parts: A, B, C and D. When your initial enrollment window ends, you can only sign up for Medicare Part A and B during the general annual enrollment period from January 1–March 31, with coverage effective July 1.
And you can sign up for Part D during the annual enrollment period from October 15–December 7, with coverage effective January 1 of the coming year.
So if you delay your enrollment, you could be paying higher premiums when you eventually do enroll, and you’ll have to wait until an open enrollment period in order to have access to coverage. If you’re only enrolled in Part A, for example, and you get diagnosed with a serious illness in April, you’ll have to wait until the following January to have Part D coverage, and until the following July—more than a year in the future—to have Part B coverage.
Although Medigap plans don’t have late enrollment penalties, insurers in most states are allowed to use medical underwriting if you apply for a Medigap plan after your initial enrollment window (when you’re first eligible for Medicare) ends. This means they can charge higher premiums or reject the application altogether if your medical history doesn’t meet their requirements. There is no annual open enrollment window for Medigap plans, so unless you’re in one of a handful of states that have guaranteed-issue rules for Medigap plans, you might be unable to purchase Medigap coverage if you don’t do so during your initial enrollment period when coverage is guaranteed-issue.