Medicare Enrollment Periods Explained: IEP, GEP & the SEP Trap
Medicare has three enrollment windows for Part B. COBRA and retiree coverage do not extend any of them, and people who assume they do end up with a penalty for life.
The Initial Enrollment Period (IEP) is the 7 months around your 65th birthday. Since 2023, coverage starts the month after you sign up, whichever month you choose. If you are still working and covered by an employer plan, you can delay Medicare and use a Special Enrollment Period (SEP) later. The SEP lasts 8 months and starts when the job or the active employer coverage ends. COBRA and retiree coverage do not count as active coverage and do not extend it. If you miss both, the General Enrollment Period (GEP) runs January 1 to March 31 each year, usually with a permanent late penalty.
Our Medicare Enrollment Period Finder asks a few questions and tells you which window applies to you. This article explains the rules behind it, and the two assumptions that most often lead to a missed deadline.
The Initial Enrollment Period: 7 months, and the coverage-gap rule that changed in 2023
Your IEP covers the 3 calendar months before the month you turn 65, your birthday month, and the 3 months after. Someone who turns 65 in June has an IEP from March 1 to September 30.
People who qualify through disability have a different IEP. Theirs is built around the 25th month of Social Security Disability Insurance (SSDI) benefits.
Before 2023, signing up in the last 3 months of the window delayed your coverage by up to 3 more months. A CMS rule that took effect January 1, 2023 ended that delay. Now coverage starts on the first day of the month after you enroll, in any month of the IEP. If you enroll in the seventh month, you are covered the next month.
The Special Enrollment Period and the COBRA trap
If you work past 65 and have health coverage through your own or your spouse's current employer, you generally do not have to enroll during the IEP. When that job or coverage ends, you get an 8-month Special Enrollment Period to sign up for Part B with no penalty.
The mistake is in what counts as employer coverage. COBRA and retiree health plans are not coverage based on current active employment. Neither one adds a day to the 8 months. The clock starts when active employment, or the active group plan, ends. It runs whether or not you elect COBRA or have retiree benefits.
A person who leaves a job, takes 18 months of COBRA and plans to sort out Medicare when COBRA ends has 8 months, not 18.
24/7 Wall St. reported a case like this in June 2026. A 64-year-old engineer took a buyout and elected 18 months of COBRA the same week. He turned 65 eleven months later, still on COBRA. When he applied for Part B, Social Security told him his 8-month window had started the month his job ended and had already closed. He owed a late-enrollment penalty for the months he had only COBRA. The next month, the same outlet reported a second person who made the same mistake with retiree coverage from a former employer. In both cases the penalty lasts as long as the person has Part B.
Initial Enrollment
7 months: 3 before your birthday month, your birthday month, and 3 after. Coverage starts the month after you enroll, whichever month you use, under a 2023 rule change.
Special Enrollment
8 months, starting when active employment or active employer coverage ends. COBRA and retiree coverage do not extend it.
General Enrollment
January 1 to March 31 every year, the fallback if you missed both other windows. Coverage starts the month after enrollment.
Late Penalty
10% of the standard Part B premium for each full 12-month period you were eligible but not enrolled, added for as long as you have Part B.
The other trap: the size of your employer
Whether it is safe to delay Part B while working also depends on how many people your employer has.
| Employer size | Who pays first at 65 | What to do about Part B |
|---|---|---|
| 20 or more employees | The employer plan. Medicare is secondary. | You can generally delay Part B while actively working and use the SEP when the job ends. |
| Fewer than 20 employees | Medicare, whether or not you have enrolled. | Most people need to enroll in Part B at 65, even while still working. |
With a small employer, the plan is allowed to pay claims as if Medicare had already paid its share. If you skipped Part B, the plan may pay only a small remainder of a large bill, or nothing. Ask your benefits administrator in writing whether the plan is primary or secondary to Medicare before you decide to delay.
The General Enrollment Period
If you miss your IEP and have no SEP, you can enroll from January 1 to March 31 of any year. The 2023 rule change also fixed the start date here. Coverage now begins the first of the month after you enroll. Under the old rule, everyone waited until July 1.
The penalty still applies. The Part B late-enrollment penalty is 10% of the standard premium for every full 12-month period you could have had Part B and did not. It is added to your premium for as long as you have Part B.
At the 2026 standard premium of $202.90 a month, one missed 12-month period adds $20.29 a month, about $243 a year. Two missed periods add about $487 a year. The dollar amount rises whenever the standard premium rises. Our Late Enrollment Penalty Calculator works out the cost for your own gap.
Before you delay Medicare, check three things
- Is your coverage from a current job, yours or your spouse's? COBRA, retiree plans and severance coverage do not count.
- Does the employer have 20 or more employees?
- What date does the active coverage end? Count 8 months from there and enroll well before the end.
Run the Medicare Enrollment Period Finder again whenever your job or coverage changes.
Frequently Asked Questions
What is the Medicare Initial Enrollment Period?
It is a 7-month window around your 65th birthday: the 3 months before your birthday month, your birthday month, and the 3 months after. Since a 2023 rule change, coverage starts the first of the month after you enroll, whichever month of the window you sign up in.
Does COBRA extend my Medicare Special Enrollment Period?
No. The 8-month Special Enrollment Period starts when your active employment or active employer group coverage ends, not when COBRA ends. Electing COBRA does not give you more time to enroll in Medicare. Waiting for COBRA to run out is a common cause of a permanent late-enrollment penalty.
Does retiree health coverage count as active employer coverage for Medicare enrollment?
No. Retiree coverage is treated like COBRA. It is not coverage based on current active employment, so it does not extend or delay your Special Enrollment Period. The 8 months start when your active employment ends.
Why does my employer's size matter for Medicare enrollment?
If your employer has 20 or more employees, the employer plan pays first and Medicare is secondary, so you can generally delay Part B while actively working and use your Special Enrollment Period later. If your employer has fewer than 20 employees, Medicare pays first at 65 whether or not you enroll. The employer plan may then pay very little on claims if you skip Part B, so most people in this situation need to enroll at 65.
What happens if I miss my Initial and Special Enrollment Periods?
You can enroll during the General Enrollment Period, January 1 through March 31 each year, and coverage starts the first of the month after you enroll. You will usually owe a permanent Part B late-enrollment penalty of 10% of the standard premium for every full 12-month period you were eligible but not enrolled.
This article explains general federal Medicare enrollment rules as of October 2026 and is not insurance, legal or financial advice. Employer size, COBRA elections and disability-based eligibility affect which window and deadline apply to you. For your own enrollment status, contact the Social Security Administration, your State Health Insurance Assistance Program (SHIP) or medicare.gov.