Medicare enrollment periods, explained without the alphabet soup

Medicare runs on deadlines, and the penalties for missing them are permanent rather than temporary. That is unusual and it is why this topic is worth twenty minutes of your attention once, rather than a panicked afternoon later.

Your Initial Enrollment Period (IEP)

This is your first window and it is seven months long: the three months before the month you turn 65, your birthday month, and the three months after. Within it you can enroll in Part A and Part B, pick up a Part D plan, choose a Medicare Advantage plan, or buy a Medigap policy.

When your coverage begins depends on when in the window you act. Enrolling before your birthday month generally starts coverage on the first of your birthday month. Enrolling later pushes the start date back, which can leave a gap. If you have a procedure planned or a prescription you cannot pause, that timing matters.

The Annual Enrollment Period (AEP) — 15 October to 7 December

This is the big one, and it is the reason your mailbox fills up every autumn. Between 15 October and 7 December you can switch Medicare Advantage plans, move between Advantage and Original Medicare, join or change a Part D plan, or drop Advantage and return to Original Medicare. Changes take effect on 1 January.

Here is the part people skip: your current plan can change underneath you for the coming year. The network can change, the formulary can change, the copays and other benefits can change. Every plan sends an Annual Notice of Change in September explaining exactly what is different. Almost nobody reads it, and that is precisely how a plan that worked last year quietly stops working.

The Medicare Advantage Open Enrollment Period (OEP) — 1 January to 31 March

If you are on a Medicare Advantage plan on 1 January, you get one more chance. Between January and the end of March you may switch to a different Advantage plan, or drop Advantage and return to Original Medicare with a standalone Part D plan.

It is a narrower window than AEP: one change only, and it is not open to people on Original Medicare who want to move into an Advantage plan. Think of it as a correction window for a January that is not going the way you expected.

Special Enrollment Periods (SEP)

Life events open windows outside the normal calendar. The common ones:

• Losing employer or union coverage — an eight-month window for Part B, and a separate shorter window for Part D and Advantage plans
• Moving out of your plan's service area, including a move between counties or states
• Your plan leaving the market or losing its Medicare contract
• Qualifying for Extra Help or Medicaid, or losing that qualification
• Moving into, living in, or moving out of a skilled nursing facility
• A qualifying five-star plan available in your area — a once-a-year switch
• This is not a complete list of Special Enrollment Periods. Visit Medicare.gov for the full list.

The General Enrollment Period (GEP) — 1 January to 31 March

This is the safety net, and you do not want to need it. If you missed your Initial Enrollment Period and have no Special Enrollment Period available, you can enroll in Part B between January and March, with coverage starting the month after you enroll.

The cost of arriving here is a Part B late-enrollment penalty: 10% added to your premium for each full 12-month period you could have had Part B and did not. That surcharge is permanent — you pay it for as long as you have Part B.

The two penalties worth knowing about

Part B: 10% added to your monthly premium for every full year you delayed without creditable coverage, for life.

Part D: roughly 1% of the national base beneficiary premium for each month you went without creditable drug coverage after eligibility, added to your premium for life. Sixty-three days is the threshold that starts the clock.

Both are avoidable with a single conversation at the right time. Neither can be undone afterwards, which is what makes them worth planning around.

What to do and when

• Three months before you turn 65: start the conversation, whether or not you plan to retire.
• Still working at 65 with employer coverage: confirm whether that coverage is creditable before you delay anything.
• Every September: read the Annual Notice of Change your plan sends. If nothing looks different, confirm that with someone.
• 15 October to 7 December: review your plan for the coming year even if you intend to keep it.
• Any major life change: ask whether it opened a Special Enrollment Period. It often has.

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The bottom line

You do not need to memorise the acronyms. You need one person who tracks the calendar with you and calls before a window closes rather than after. That is a large part of what Sarah does for the people she works with.