Medicare Enrollment Periods Explained: IEP, AEP, and SEPs

By Naomi Foster, Contributing Writer, Healthcare
Updated July 2, 2026

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

Details: The Initial Enrollment Period (IEP)

The IEP is the first window 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. For example, if you turn 65 in July, your IEP runs from April 1 through October 31.

When your coverage begins depends on when you sign up within that window. Enroll in the three months before your birthday month and coverage generally starts on the first day of your birthday month. Enroll during or after your birthday month and coverage is delayed by one or more months. Enrolling early is usually advisable to avoid a 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. Your Medicare card arrives by 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

If you are happy with your current plan, you do not have to do anything. But reviewing each fall is worthwhile because plan costs, formularies, and networks can change. 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 window runs from January 1 through March 31 each year. People already enrolled in a Medicare Advantage plan can use it to switch to a different Advantage plan or return to Original Medicare and join a standalone Part D plan. You may make only one change during this period, and it cannot be used to switch from Original Medicare into Medicare Advantage.

Special Enrollment Periods (SEPs)

SEPs let you enroll in or change Medicare coverage outside the standard windows when certain qualifying life events occur. The most common 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 or group coverage ends, whichever comes first. This SEP gives you eight months to enroll in Part B without penalty. COBRA and retiree coverage do 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 outside your current plan's service area, or to a location with new plan options, triggers a SEP to join a plan in your new area.

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 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. 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 the dollar amount 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.

Things to Know: 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 the free Medicare Advantage cost calculator to estimate what different plan choices might cost you, and confirm your specific eligibility and timing with Medicare directly or with a free SHIP counselor. Official guidance is at Medicare.gov.

Related Reading

• comparing Advantage to Original Medicare
• the full 2026 cost breakdown