9.1 Medicare Enrollment Periods

Medicare Enrollment Periods: The Five Windows

Quick Answer: Medicare enrollment is governed by five distinct time windows. The Initial Enrollment Period (IEP) spans 7 months around your 65th birthday (3 before, the month of, and 3 after). The General Enrollment Period (GEP) runs January 1 to March 31 each year for those who missed the IEP, with coverage starting the month after you enroll and a permanent late penalty. The Annual Enrollment Period (AEP) runs October 15 to December 7 for changing Medicare Advantage and Part D plans. The Medicare Advantage Open Enrollment Period runs January 1 to March 31 for one switch. Special Enrollment Periods (SEPs) apply to specific life events such as still working with employer coverage.

Initial Enrollment Period (IEP)

The Initial Enrollment Period (IEP) is the first and most important enrollment window. It lasts 7 months: the 3 months before your 65th birthday month, your birthday month, and the 3 months after. If your birthday falls on the first of the month, Medicare shifts the window back one month, so coverage can start up to a month earlier than it otherwise would.

If you enroll during the first 3 months, coverage generally begins the first day of your birthday month. If you enroll during your birthday month, coverage starts the next month. If you enroll in one of the 3 months after, coverage starts 1 to 3 months later, depending on which month you choose. Delaying past the IEP without qualifying coverage triggers late penalties.

General Enrollment Period (GEP)

The General Enrollment Period (GEP) runs January 1 through March 31 every year. It is the safety net for people who did not enroll during their IEP and do not qualify for an SEP. Coverage begins the month after you sign up, so a January 15 enrollment means March 1 coverage.

If you enroll in Part B during the GEP because you missed your IEP, you owe the Part B late enrollment penalty: 10% of the monthly premium for each full 12-month period you were eligible but not enrolled. The penalty is permanent and stacks. Someone 24 months late pays 20% extra for as long as they have Part B.

Annual Enrollment Period (AEP / Open Enrollment)

The Annual Enrollment Period (AEP) runs October 15 through December 7 each year. This is the only window most beneficiaries use annually. During the AEP you can:

• Switch from Original Medicare to a Medicare Advantage plan
• Switch from a Medicare Advantage plan back to Original Medicare
• Change from one Medicare Advantage plan to another
• Join, drop, or switch a standalone Part D prescription drug plan

All changes take effect January 1 of the following year. The AEP does not let you change Medigap policies, which have their own enrollment rules.

Medicare Advantage Open Enrollment Period

From January 1 through March 31, anyone already enrolled in a Medicare Advantage plan has a one-time chance to:

• Switch from Medicare Advantage to Original Medicare (with or without a Part D plan), or
• Switch to a different Medicare Advantage plan

You get one switch during this window. If you disenroll from an MA plan that includes drug coverage and go to Original Medicare, you must use a Special Enrollment Period or wait for the next AEP to add a standalone Part D plan, or you risk a late penalty.

Special Enrollment Periods (SEPs)

Special Enrollment Periods let you enroll or change coverage outside the standard windows when a qualifying life event occurs. The most commonly tested SEP is the active-employment SEP: if you (or your spouse) are still working and covered by a group health plan based on that active employment, you can enroll in Part B any time while covered, and for up to 8 months after that coverage ends, without a late penalty. This 8-month SEP does not apply to Part D in the same way.

Other SEPs include:

• Moving out of your plan's service area
• Losing other creditable coverage (for example, employer coverage ends)
• Losing Medicaid eligibility
• Returning to the United States after living abroad
• Having a plan leave your area or a contract terminated

Medigap Open Enrollment

Medigap (Medicare Supplement) policies have a unique 6-month open enrollment period that begins the month you are both 65 or older and enrolled in Part B. During this window, issuers must sell you any Medigap policy available in your state with no medical underwriting and no pre-existing condition exclusions. Outside this window, in most states Medigap can be medically underwritten and denied, so this is the key window to lock in coverage.

Part D Late Enrollment Penalty

If you go 63 or more consecutive days without creditable prescription drug coverage (Part D or equivalent) after your IEP ends, you owe the Part D late enrollment penalty. The 63-day gap is the trigger, not the multiplier: once it is crossed, the penalty is 1% of the national base beneficiary premium multiplied by the number of full, uncovered months you went without creditable drug coverage, rounded to the nearest $0.10 and added permanently to your monthly premium. A 14-month gap therefore produces a 14% penalty. The Part B penalty is measured in full 12-month periods; the Part D penalty is measured in full months once the 63-day threshold has been crossed.

Enrollment Periods at a Glance

Exam tip: The most frequently tested combination is the IEP (7 months around the 65th birthday) and the 8-month SEP for active workers. Remember the first-of-the-month birthday rule, which shifts the IEP back one month, and remember that the Part B penalty is measured in full 12-month periods while the Part D penalty is triggered by a 63-day gap and then counted in full uncovered months.