Millions of Americans use Medicare as their primary insurance. However, some expenses are not covered if you have original Medicare, also known as Parts A and B. As a result, you may have to pay out of your pocket for these, which can add up. Medigap, a type of supplemental insurance, is designed to help cover some of the costs not paid for by Medicare.

However, you can't simply sign up for Medigap whenever you'd like. As with original Medicare, there is a Medigap open enrollment period. So if you're considering Medigap supplemental coverage, you'll want to make sure you know exactly when your enrollment period is to sign up.

What is Medigap?

Medigap is a type of Medicare supplemental insurance. It's private health insurance that works with original Medicare. Original Medicare comprises two parts: Part A covers hospital visits, and Part B covers doctor's visits. However, Medicare doesn't pay for all of your health care costs; it typically covers about 80% of related expenses. The rest you have to pay out of pocket.

Medigap helps cover some of these remaining expenses. When you pick a plan, you'll pay a monthly premium, just as you would with traditional employer-provided healthcare. Then, based on the policy you choose, your insurer will pay for those additional out-of-pocket expenses Medicare doesn't cover.

What is Medigap Open Enrollment?

You must be enrolled in Medicare Part B to sign up for Medigap. Typically, people who choose to sign up for Medigap do it at the same time they enroll in Medicare. The Medigap open enrollment period is a seven-month window that runs alongside your Medicare enrollment period. It starts three months before the first day of the month you will turn 65 and follows for three months after. Depending on your state, there may be additional open enrollment periods. If you're unsure if that's the case in your state, contact Medicare directly to find out more.

If you're considering Medigap coverage, there's one significant factor to understand before you sign up. Your Medigap coverage is guaranteed if you enroll in Medigap during the open enrollment period. However, once that open enrollment period passes, insurers have the right to deny you Medigap coverage. Once the open enrollment period ends and that guarantee lapses, insurers can review your medical history and reject you based on your health or any pre-existing conditions.

There are a handful of other scenarios in which you may still qualify for guaranteed Medigap coverage if you missed your initial open enrollment period. These include:

• When you turned 65, you joined Medicare Advantage, and then after one year, you decided to switch to original Medicare.
• You've moved out of your service area for Medicare Advantage.
• Your Medicare Advantage plan or your current Medigap plan shut down.
• If you had an employer plan that supplemented your Medicare coverage, the program ended or you are no longer employed in that role anymore.

How to Find a Policy That Works for You

As you approach 65, you want to start thinking about your health insurance plans — especially if you plan to use a combination of original Medicare and Medigap for your coverage.

There are ten standardized Medigap plans. Each is the same across every state, though not all states will offer all the available plans.

If you decide to go this route, here's how to get started:

• Review the plans offered in your area.
• Pick the policy that looks best for you.
• Call the insurer and get a quote for the policy.
• Once you're ready to buy, purchase your plan.

Remember, doing this within the open enrollment window will guarantee your coverage. However, if you decide to buy a Medigap plan later, you may not have that same guarantee.

Deciding on your healthcare in retirement can feel overwhelming. But the key is to gather as much information as you can before you need to make any major decisions. Then, if you have questions, speak with a financial professional about the options you think may work best for you or contact Medicare directly and talk to a specialist who can answer your questions.