Medicare Enrollment Periods Explained: IEP, AEP, and SEPs

Medicare has several distinct enrollment windows, and which one applies to you depends on your age, your current coverage, and your life circumstances. The three main types are the Initial Enrollment Period (IEP) when you first become eligible, the Annual Enrollment Period (AEP) every fall, and various Special Enrollment Periods (SEPs) triggered by qualifying life events. Missing deadlines without a valid exception can result in late enrollment penalties that last for as long as you have Medicare.

The Initial Enrollment Period (IEP)

The IEP is the first window you have to sign up for Medicare. It lasts seven months: it begins three months before the month you turn 65, includes your birthday month, and extends three months after your birthday month. For example, if you turn 65 in July, your IEP runs from April 1 through October 31.

When your coverage begins depends on when within the IEP you sign up. If you enroll in the three months before your birthday month, your coverage generally starts on the first day of your birthday month. If you enroll during your birthday month or in the three months after, coverage is delayed by one or more months. Enrolling early in your IEP is usually advisable to avoid a coverage gap.

The IEP covers enrollment in Part A (hospital insurance), Part B (medical insurance), Medicare Advantage (Part C) plans, and Part D drug plans. You can find official enrollment guidance at the Social Security Administration's Medicare benefits page, since most people sign up through Social Security.

Automatic Enrollment

If you are already receiving Social Security retirement benefits when you turn 65, you are generally enrolled in Part A and Part B automatically. You will receive your Medicare card in the mail about three months before your 65th birthday. If you do not want Part B (for example, because you have employer coverage), you must actively opt out before your coverage begins.

The Annual Enrollment Period (AEP)

The AEP runs every year from October 15 through December 7. Changes made during the AEP take effect on January 1 of the following year. This is the main window for beneficiaries already enrolled in Medicare to make plan changes.

During the AEP, you can:

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

The AEP is one of the most important annual deadlines in Medicare. If you are happy with your current plan, you do not have to do anything. But reviewing your plan each fall is worthwhile because plan costs, formularies, and networks can change from year to year. Plans must send you an Annual Notice of Change (ANOC) by September 30 each year outlining what will be different in the coming plan year.

You can compare plans during the AEP at Medicare's official Plan Finder.

The Medicare Advantage Open Enrollment Period (MA OEP)

A separate and often overlooked enrollment window runs from January 1 through March 31 each year. During this period, people who are already enrolled in a Medicare Advantage plan can switch to a different Medicare Advantage plan or drop their Medicare Advantage plan and return to Original Medicare (and join a standalone Part D plan). You may only make one change during this period. This window cannot be used to switch from Original Medicare to Medicare Advantage.

Special Enrollment Periods (SEPs)

SEPs allow you to enroll in or change Medicare coverage outside the standard windows when certain qualifying life events occur. The most common SEPs include:

Employer Coverage SEP

If you delayed Medicare enrollment at 65 because you or your spouse had health coverage through active employment, you qualify for a SEP when that employment ends or the group coverage ends, whichever comes first. This SEP gives you eight months to enroll in Part B without penalty. Note that COBRA or retiree coverage does not count as active employer coverage for this purpose, so the SEP clock starts when the active employment coverage ends, not when COBRA ends.

Moving SEPs

Moving to a new address that is outside your current plan's service area, or moving somewhere with new plan options, triggers a SEP to join a plan in your new location.

Other Common SEPs

• Losing other creditable coverage involuntarily (such as losing Medicaid eligibility)
• A plan leaving the Medicare program or losing its Medicare contract
• Qualifying for Extra Help with drug costs (Low Income Subsidy)
• Entering or leaving a skilled nursing facility or nursing home

The rules for each SEP are specific. For a complete list, visit Medicare.gov's Special Enrollment Periods page.

Late Enrollment Penalties

Failing to enroll in Medicare at the right time without a qualifying SEP can result in permanent premium surcharges. These penalties are not one-time fees. They apply every month for as long as you have coverage.

Part B Late Penalty

If you did not sign up for Part B when first eligible and did not qualify for a SEP, your monthly Part B premium increases by 10% for each full 12-month period you could have had Part B but did not enroll. For example, missing two full years adds a 20% surcharge. In 2026, with a standard premium of $202.90, a 20% penalty means paying approximately $40.58 more every month, permanently (and it adjusts upward if the standard premium rises in future years).

Part D Late Penalty

If you go 63 or more days without creditable prescription drug coverage after becoming eligible for Part D, you will owe a late enrollment penalty. The penalty equals 1% of the national base beneficiary premium multiplied by the number of uncovered months. In 2026, the national base beneficiary premium is approximately $38.99. The penalty is rounded to the nearest $0.10 and added permanently to your monthly Part D premium.

Penalty Type	How It Is Calculated	Duration
Part B late penalty	10% of standard premium per 12-month period missed	Permanent (as long as you have Part B)
Part D late penalty	1% of national base premium per month without coverage	Permanent (as long as you have Part D)
Part A late penalty (if applicable)	10% surcharge on Part A premium for twice the number of years you could have enrolled	Limited duration

When to Plan Ahead

If you are approaching 65, mark your IEP start date on your calendar three months before your birthday. If you plan to keep working past 65, confirm with your employer's benefits administrator whether your coverage qualifies as creditable and whether you need to enroll in Part A. If you are already enrolled in Medicare, set a reminder each fall to review plan options during the AEP before December 7.

Enrollment rules are detailed and the stakes are real. Use our free Medicare Advantage cost calculator to estimate what different plan choices might cost you, and always confirm your specific eligibility and timing with Medicare directly or with a free SHIP counselor. Official guidance is available at Medicare.gov.

Related reading

• MA vs Original
• MA Costs 2026