Medicare Initial Enrollment Period Mistakes to Avoid

How the Seven-Month Window Works

The Medicare Initial Enrollment Period starts three months before the month your parent turns 65, includes their birthday month, and ends three months after. That's it—seven months to enroll in Medicare Parts A and B; Medigap and other supplemental coverage have separate enrollment rules. The window is rigid, and every month within it changes when coverage actually starts.

For most people whose birthday isn't the first day of the month, enrolling during the three months before the birthday month triggers coverage on the first day of the birthday month. If the birthday is the first day of the month, the relevant month for Medicare eligibility and coverage is the prior month. Enrolling during the birthday month itself delays coverage start by one month. Enrolling one, two, or three months after pushes the effective date further out—up to three months of delay. Those gaps aren't abstract; they're months without medical coverage where any hospitalization or specialist visit comes entirely out of pocket.

Mistake 1: Assuming Part B Is Automatic

Part A (hospital coverage) is automatic if your parent is already receiving Social Security benefits. Part B (medical coverage) is not always automatic, and it's the one that matters more for daily healthcare—doctor visits, outpatient procedures, lab work, durable medical equipment.

If your parent isn't receiving Social Security yet (because they're still working or delayed claiming), they must actively sign up for Part B during the IEP. Missing it triggers the late enrollment penalty: 10% of the standard premium ($202.90/month in 2026) for every full 12-month period they could have had Part B but didn't. That penalty is permanent—it stays on their premium for life.

Mistake 2: Confusing Employer Coverage Rules

Parents who work past 65 with employer health insurance through a company with 20+ employees can delay Medicare Part B without penalty. But the rules are precise: the coverage must be through active employment (the parent's own job or their spouse's current employer), and COBRA, retiree plans, and marketplace plans do not count as creditable coverage that allows a penalty-free delay.

When employment ends, the Special Enrollment Period is eight months—not a day longer. Your parent also needs Form CMS-L564, which their employer fills out to document prior creditable coverage. Start the paperwork 60–90 days before retirement to avoid a gap.

The trap for small employers: if the company has fewer than 20 employees, Medicare becomes primary even while your parent is still working, and delaying Part B enrollment will trigger the penalty.

Mistake 3: Missing the Medigap Window

The Medigap Open Enrollment Period is tied to Part B enrollment, not the birthday. It starts the first month your parent is both 65 or older and enrolled in Part B, and it lasts exactly six months. During this window, insurers must sell any Medigap policy at the standard rate regardless of health history—no medical underwriting, no denials, no higher premiums for pre-existing conditions.

This is the only federally guaranteed window in most states. Once it closes, 31 states let insurers use medical underwriting to deny Medigap coverage entirely. A parent who skips Medigap at 65 because they're healthy may find themselves uninsurable at 72 when they've developed the conditions that make Medigap most valuable.

If your parent enrolls in Part B late—say, at 67 after leaving employer coverage—their Medigap OEP also starts late, at 67. The window tracks Part B enrollment, not age.

Mistake 4: Ignoring Part D

Part D prescription drug coverage has its own IEP that overlaps with the Medicare IEP. Skipping it—because your parent takes no medications right now—triggers a 1% monthly penalty for every month without creditable drug coverage. Twelve months without Part D means a $4.70/month penalty added permanently to future premiums.

Even if your parent is healthy and takes nothing, enrolling in a low-premium Part D plan during the IEP avoids the penalty entirely. It's cheap insurance against a future need.

Mistake 5: Not Coordinating Timing

The ideal sequence starts enrollment in the first three months of the IEP—before the birthday month for most people—so coverage begins on the first day of the relevant eligibility month with no gap. But families routinely start the process late, submit paperwork in the birthday month, and end up with a one- to three-month coverage gap while everything processes.

For families juggling employer coverage transitions, the timing gets trickier. The employer Special Enrollment Period can overlap with the IEP, and the Medigap OEP depends on when Part B starts. Getting the sequence wrong doesn't just delay coverage; it can permanently change the premiums your parent pays.

Our Medigap vs Medicare Advantage guide includes a detailed enrollment timeline with penalty calculations, deadline trackers, and a coordination worksheet for families managing the transition from employer coverage to Medicare.

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