Social Security benefits make up about a third of the average retiree's monthly income, according to the Social Security Administration. But close to half of unmarried beneficiaries rely on Social Security for at least 90% of their income.

It certainly makes sense to take Social Security into consideration while you're planning for retirement. In fact, Social Security benefits are designed to replace roughly 40% of your income during retirement. But if your retirement fund is smaller than you'd hoped and you're planning on relying on Social Security to make ends meet, you may be in for a rude awakening. There are two major reasons you shouldn't rely too much on your benefits during retirement.

Elderly couple sitting on a bench at the beach

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1. You may see a slash in benefits in the future

Seventy-eight percent of people are worried Social Security will run dry in their lifetime, according to Nationwide's annual retirement survey. While the program won't completely run out of money, retirees may see cuts in the relatively near future.

There are roughly 10,000 baby boomers retiring each day, and there simply aren't as many younger workers generating payroll taxes. In other words, there's more money coming out of the system than there is going in. Life expectancies are also increasing, with one in four people turning 65 today expected to live past age 90, and one in 10 making it past 95. And the more years people spend in retirement, the more benefits they'll receive.

As a result, the $2.8 trillion in asset reserves is expected to be depleted by 2034, according to the most recent Social Security Board of Trustees report. Again, this doesn't mean that the Social Security program will be broke by 2034; it just means there may be cuts. According to the trustees, benefits may need to be cut by up to 23% in order for the program to remain stable over the next 75 years.

Keep in mind, however, that it may not come to this. Hopefully, Congress will come up with a solution before retirees start seeing their benefits slashed. But it never hurts to be careful. While you may not have much control over whether or not your benefits are cut, you can protect yourself by maximizing them.

Just how much you'll receive in benefits depends on when you start claiming them. If you claim benefits at 62, you'll receive less than you would if you wait until your full retirement age (67 if you were born in 1960 or later), and if you wait until 70, you'll receive even fatter benefits checks. So, for example, if your full retirement age is 67 and you start claiming benefits at age 70, you'll receive 124% of your monthly benefit. Even if benefits are cut by 23%, you'll have a built-in buffer to protect your savings.

2. Social Security alone (probably) won't cover your healthcare bills

Healthcare is likely one of the biggest expenses you'll face during retirement. If you're relying on Social Security to pay the bills, your monthly check may not cut it.

There's a common misconception among retirees that Medicare will take care of all your medical bills once you turn 65. While Medicare will help with some healthcare expenses, you'll still be responsible for deductibles, copayments, and coinsurance. Medicare also doesn't cover most dental care, prescription eyeglasses, or long-term care -- all of which can come with hefty bills.

The average 65-year-old couple can expect to pay about $275,000 in healthcare bills during retirement, according to a recent Fidelity study. If your retirement lasts 20 years, that comes down to $13,750 per year for a couple, or $6,875 per person. With the average Social Security check is around $1,400 per month (totaling $16,800 per year), that means you may end up spending 40% of your benefits on healthcare alone.

That may not seem too bad, but those figures don't take long-term care into consideration. Around 70% of people turning 65 today will need long-term care (such as nursing home or assisted living care) at some point in their lifetime, and it's not cheap. A semi-private room in a nursing home, for instance, can cost over $6,800 per month -- or over $82,000 per year. Medicare also doesn't typically cover nursing home care unless it's medically necessary, and Social Security alone won't be enough to cover the expenses either.

So what can you do, then, to prepare yourself for these costs? You have a couple options. First, you can open a health savings account (HSA) to start building a savings fund just for your medical bills. With an HSA, your contributions are tax-deductible upfront, and you also don't need to pay taxes when you withdraw if the money is going toward eligible medical expenses. This can help cover deductibles, copays, and other medical expenses, and it also acts as a second retirement fund because once you turn 65, you can withdraw the money penalty-free even for non-medical expenses (you'll just need to pay income tax on it).

Long-term care insurance is another option if you're concerned about high healthcare bills. Premiums are notoriously high for this type of insurance (especially the longer you wait to sign up) and may cost thousands of dollars per year. But if you're expecting you'll need long-term care (or if you just want to play it safe), it may be a good investment.

You never know exactly how much you'll need to save for retirement, which is why planning for it is so challenging. While Social Security can help pay your bills and provide a steady income once you leave the workforce, it isn't designed to replace your income entirely. To make sure you're as prepared as possible, it's a good idea to have a backup plan in case your Social Security check isn't quite as bountiful as you'd hoped.