What is the Medicare Initial Enrollment Period?
Your birthday month sits in the middle of Medicare's first sign-up window, and the month you sign up in sets when coverage starts and whether a late penalty can follow.
Arun Verma, co-founder of Triangle · 2026-10-17
This is not clinical evidence and not medical advice. It does not replace your care team. Discuss anything here with your clinician before you act on it.
What is the Medicare Initial Enrollment Period?
It is your first chance to sign up for Medicare, and for most people it is a seven-month window around the 65th birthday. The window starts three months before the month you turn 65, includes your birthday month, and ends three months after it. Medicare.gov describes it as generally when you turn 65. Coverage always starts on the first of a month, and which month depends on when you sign up inside the window. This page covers the age-65 window. CMS says the period is counted differently for people who become entitled to Medicare before turning 65.
When does Medicare coverage start if I sign up during the window?
It starts on the first of a month, and the month depends on when you sign up. For Part B, and for Part A if you pay a premium for it, Medicare.gov says signing up before your birthday month starts coverage the month you turn 65. Signing up in your birthday month, or in any of the three months after, starts it the following month. Premium-free Part A follows a different rule: it starts the month you turn 65, or the month before if your birthday is on the first of the month. SSA's page on when to sign up has a spot to enter your birth date and see when you can first enroll.
Am I enrolled automatically, or do I have to sign up?
It depends on whether you already get Social Security (or Railroad Retirement Board) benefits. People who start getting them at least four months before turning 65 are generally enrolled in Part A and Part B automatically, and a welcome package with a Medicare card is mailed three months before coverage starts. If you live in Puerto Rico, CMS says automatic enrollment covers only premium-free Part A, and you have to sign up for Part B yourself. CMS adds that people who are enrolled automatically can choose whether to keep Part B. Without those benefits, you apply. CMS says people who aren't receiving Social Security or Railroad Retirement Board benefits aren't automatically enrolled and must apply through Social Security. SSA's sign-up page says people 65 or older can enroll in Parts A and B online.
What happens if I miss the Initial Enrollment Period?
You may have to wait for another sign-up period and pay a monthly late enrollment penalty for as long as you have Part B, and the penalty goes up the longer you wait. After the window closes, Part B and premium Part A are open only during the other enrollment periods. One of those is the General Enrollment Period, January 1 to March 31 each year, with coverage starting the month after you sign up. CMS says the Part B premium may go up 10% for each full 12-month period you could have had Part B but didn't sign up. Medicare.gov says the penalty is added to your monthly premium rather than charged once, and that people who have to buy Part A face a separate one: a premium that may go up 10%, paid for twice the number of years they didn't sign up. Exceptions exist, and the next question covers them.
When does a Special Enrollment Period apply instead?
Sometimes you can sign up after the window without a late penalty. That is a Special Enrollment Period, and it is available only in certain situations and for a limited time. It covers Part B and premium Part A. One situation involves job-based coverage. If you or your spouse have health insurance through a job, SSA says you can sign up any time while you're working and still covered, or within 8 months of when work or the group plan ends. Not every kind of coverage counts. Medicare.gov says the end of COBRA or retiree coverage doesn't open a Special Enrollment Period, and neither does having or losing Marketplace coverage. Other Special Enrollment Periods cover specific events, such as a declared natural disaster that caused you to miss the window (for events on or after January 1, 2023). CMS says the Special Enrollment Period for "other exceptional conditions" is only granted when the conditions are truly exceptional, not for forgetfulness or lack of knowledge. Whether your own coverage counts is a fair question for your benefits administrator or Social Security.
How is it different from fall Open Enrollment?
The Initial Enrollment Period is for signing up for the first time. Open Enrollment is for changing coverage you already have. It runs October 15 to December 7 each year, with changes taking effect January 1 of the next year. During Open Enrollment you can join, drop or switch a Medicare Advantage Plan (with or without drug coverage), move between Original Medicare and Medicare Advantage, and, if you're in Original Medicare, join, drop or switch a drug plan. Medicare.gov says you choose a health or drug plan once you have Part A and/or Part B. For Part B and premium Part A themselves, CMS's list of sign-up periods is the Initial, General and Special Enrollment Periods. Open Enrollment isn't on it.
Does the Initial Enrollment Period also cover joining a plan?
Yes, it is also a window for joining a Medicare health or drug plan, provided you have the right parts. Medicare.gov says you need both Part A and Part B to join a Medicare Advantage Plan, and either one to join a drug plan. If you join a Medicare Advantage Plan during this window, you can switch to another one or go back to Original Medicare within the first three months you have both Part A and Part B. Plan timing follows your Part A and B start dates: if you ask to join before they start, plan coverage starts the same day, and if you ask after, it starts the first of the month after the plan gets your request.
How do I sign up, and what information is needed?
You sign up through Social Security, by phone or, if you're 65 or older, online. SSA lists the basics it asks for: your Social Security number, where you were born, and the start and end dates of any current group health plan and any group plan after age 65. You could look up those group plan dates before you start.
Sources
- Medicare.gov: When does Medicare coverage start? (page carries no date): Medicare.gov says the Initial Enrollment Period is generally when you turn 65 and lasts 7 months, starting 3 months before you turn 65 and ending 3 months after the month you turn 65.
- CMS: Original Medicare (Part A and B) Eligibility and Enrollment (page carries no date): CMS says the Initial Enrollment Period is a 7-month period that begins 3 months before the month a person turns 65, includes their birthday month and ends 3 months after the person turns 65.
- Medicare.gov: When does Medicare coverage start? (page carries no date): Medicare.gov says Medicare coverage always starts on the first of the month, and the date depends on which month you sign up during your Initial Enrollment Period.
- CMS: Original Medicare (Part A and B) Eligibility and Enrollment (page carries no date): CMS says that for someone under age 65 who becomes entitled to Medicare, the Initial Enrollment Period is counted from a different starting point, and that the period varies by situation for some groups.
- Medicare.gov: When does Medicare coverage start? (page carries no date): Medicare.gov says that for Part B (and premium-Part A), signing up before the month you turn 65 starts coverage the month you turn 65, and signing up the month you turn 65 or during the 3 months after starts coverage the next month.
- Medicare.gov: When does Medicare coverage start? (page carries no date): Medicare.gov says premium-free Part A coverage starts the month you turn 65, or the month before you turn 65 if your birthday is on the first of the month.
- SSA: When to sign up for Medicare (page carries no date): SSA says you can enter your birth date on its page to see when you can first enroll.
- Medicare.gov: I’m getting Social Security benefits before 65 (page carries no date): Medicare.gov says that if you get Social Security benefits at least 4 months before you turn 65, you are automatically enrolled in Part A and Part B, and a welcome package with your Medicare card is mailed 3 months before your coverage starts.
- CMS: Original Medicare (Part A and B) Eligibility and Enrollment (page carries no date): CMS says people who are automatically enrolled can choose whether to keep or refuse Part B.
- CMS: Original Medicare (Part A and B) Eligibility and Enrollment (page carries no date): CMS says individuals already receiving Social Security or Railroad Retirement Board benefits at least 4 months before being eligible for Medicare, and living in the United States (except residents of Puerto Rico), are automatically enrolled in both premium-free Part A and Part B, and that people living in Puerto Rico who are eligible for automatic enrollment are only enrolled in premium-free Part A and must actively enroll in Part B.
- CMS: Original Medicare (Part A and B) Eligibility and Enrollment (page carries no date): CMS says individuals who are not receiving Social Security or Railroad Retirement Board benefits are not automatically enrolled, and that an individual who is not receiving those benefits must file an application for Medicare by contacting the Social Security Administration.
- SSA: Sign up for Medicare (page carries no date): SSA says you'll sign up for Medicare Part A and Part B through Social Security.
- SSA: Sign up for Medicare (page carries no date): SSA says that if you're 65 or older, you can enroll online for Parts A and B, or Part A only.
- Medicare.gov: When does Medicare coverage start? (page carries no date): Medicare.gov says that if you miss your 7-month Initial Enrollment Period, you may have to wait to sign up and pay a monthly late enrollment penalty for as long as you have Part B coverage, and that the penalty goes up the longer you wait.
- Medicare.gov: When does Medicare coverage start? (page carries no date): Medicare.gov says that after your Initial Enrollment Period ends, you can only sign up for Part B and premium-Part A during one of the other enrollment periods.
- Medicare.gov: When does Medicare coverage start? (page carries no date): Medicare.gov says the General Enrollment Period runs January 1-March 31 each year, coverage starts the month after you sign up, and you might pay a monthly late enrollment penalty if you don't qualify for a Special Enrollment Period.
- CMS: Original Medicare (Part A and B) Eligibility and Enrollment (page carries no date): CMS says that if an individual did not sign up for Part B when first eligible, the monthly premium for Part B may go up 10% for each full 12-month period the individual could have had Part B but did not sign up for it.
- Medicare.gov: Avoid late enrollment penalties (page carries no date): Medicare.gov says late enrollment penalties are added to your monthly premium and are not a one-time late fee.
- Medicare.gov: Avoid late enrollment penalties (page carries no date): Medicare.gov says that if you have to buy Part A and don't buy it when you're first eligible, your monthly premium may go up 10%, and you'll pay the penalty for twice the number of years you didn't sign up.
- Medicare.gov: When does Medicare coverage start? (page carries no date): Medicare.gov says that after the Initial Enrollment Period there are certain situations when you can sign up for Part B (and premium-Part A) without paying a late enrollment penalty, and that a Special Enrollment Period is only available for a limited time.
- SSA: When to sign up for Medicare (page carries no date): SSA says that if you or your spouse have health insurance through your job, you can sign up any time while working and still covered by the group health plan, within 8 months of the day you or your spouse stop working, or within 8 months of the group health plan ending while you or your spouse continue to work.
- Medicare.gov: When does Medicare coverage start? (page carries no date): Medicare.gov says these situations don't qualify for a Special Enrollment Period: your COBRA coverage or retiree coverage ends, and you have or lose Marketplace coverage.
- Medicare.gov: When does Medicare coverage start? (page carries no date): Medicare.gov lists a Special Enrollment Period for people who missed a chance to sign up because they were affected by a natural disaster or an emergency that is declared or starts on or after 1/1/2023.
- CMS: Original Medicare (Part A and B) Eligibility and Enrollment (page carries no date): CMS says a request for the Special Enrollment Period for other exceptional conditions will only be granted in conditions that are truly exceptional in nature, and will not be used to grant enrollment due to forgetfulness, lack of knowledge, or failure to make premium payments.
- Medicare.gov: Open Enrollment (page carries no date): Medicare.gov says Open Enrollment happens from October 15 to December 7 and is the time each year when you can make changes to your coverage, effective January 1 of next year (the plan must get your enrollment request by December 7).
- Medicare.gov: Joining a plan (page carries no date): Medicare.gov says that during the Open Enrollment Period you can join, drop, or switch to another Medicare Advantage Plan with or without drug coverage (or add or drop drug coverage), switch between Original Medicare and a Medicare Advantage Plan, and join, drop, or switch to another Medicare drug plan if you're in Original Medicare.
- CMS: Original Medicare (Part A and B) Eligibility and Enrollment (page carries no date): CMS says the enrollment periods that apply to both premium Part A and Part B are the Initial Enrollment Period, the General Enrollment Period and a Special Enrollment Period.
- Medicare.gov: Joining a plan (page carries no date): Medicare.gov says that once you have Medicare Part A and/or Part B, you can choose a Medicare health or drug plan, and that you can only join, switch, or drop a Medicare Advantage Plan or Medicare drug plan at certain times called enrollment periods.
- Medicare.gov: Joining a plan (page carries no date): Medicare.gov says that during the Initial Enrollment Period you can join any plan, but you need both Part A and Part B to join a Medicare Advantage Plan with or without drug coverage, and either Part A or Part B to join a Medicare drug plan.
- Medicare.gov: Joining a plan (page carries no date): Medicare.gov says that if you joined a Medicare Advantage Plan during your Initial Enrollment Period, you can change to another Medicare Advantage Plan or go back to Original Medicare within the first 3 months you have Medicare Part A and Part B.
- Medicare.gov: Joining a plan (page carries no date): Medicare.gov says that if you request to join a plan before your Part A and/or Part B starts, plan coverage starts the same day your Part A and/or Part B starts, and if you request after it starts, plan coverage starts the first of the month after the plan gets your request.
- SSA: Sign up for Medicare (page carries no date): SSA lists the information you'll need to provide: your Social Security number, where you were born, and start and end dates for any current group health plans and any group health plans after age 65.
- SSA: Sign up for Medicare (page carries no date): SSA lists a phone option for signing up for Medicare Parts A and B, or Part A only: you tell the representative which you want.
Start your 7-day free trialMilepost is made by Triangle. 7-day free trial, then $9 a month. Cancel anytime.