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All the early signs are there that you might be pregnant. The sore boobs. The tiredness. The frequent trips to the bathroom to pee. Most tellingly, Aunt Flo is a no-show even though you were expecting her two days ago.

You’re frantically digging to the bottom of a bathroom drawer when you find it — a leftover unused pregnancy test from a few years ago. You breathe a sigh of relief and take it — and it’s negative. This has you wondering: Is this test still good?

The short answer is yes, home pregnancy tests — of all varieties, including digital and early response options — have expiration dates. These dates are typically stamped on the box the tests come in as well as the individual wrapping of each test. So if you find a stray test without its box, you’ll still be able to find when it expires — or expired, if it already did.

Home pregnancy tests work by detecting human chorionic gonadotropin (hCG) in the urine. This is a hormone produced by the body once an embryo implants in the uterus. If you’re not pregnant, you won’t have hCG. If you are, you’ll have levels of hCG that rapidly rise in the early days and weeks of pregnancy.

The chemical that’s used in home pregnancy tests to detect hCG is actually a trade secret. But we do know it’s an hCG antibody. The antibody reacts chemically with hCG (because that’s what antibodies do — react with certain other substances) if it’s present. The antibody then releases an enzyme that produces an additional colored line (or plus sign, or digital positive, depending on the test).

The antibody used in home pregnancy tests has a shelf life. In other words, after a certain period of time, it no longer produces that chemical reaction with hCG.

Generally, tests are good for 1 to 3 years after manufacture. A high quality, extra sensitive test may last longer than your run-of-the-mill “internet cheapie” test (as they’re called in trying-to-conceive forums; an example can be found for purchase online here). But the point is, they all have a shelf life. (The more sensitive a test, the lower the level of hCG it can detect.)

As you might expect, a home pregnancy test isn’t guaranteed to be accurate after its expiration date. If the chemical is no longer able to detect hCG — even if it’s present — it makes sense that you’d be more likely to get a false negative with an expired test. (A false negative is when you’re pregnant, but the test says you’re not.)

False positives are also possible, especially if all that time the test spent stored in your bathroom exposed it to heat and humidity. Basically, when you’re dealing with an expired test, anything goes — so we don’t recommend using one.

To get the most accurate result out of a home pregnancy test, it’s best to do the following:

At time of purchase, check the expiration date on the box. Because expiration dates may need a little wiggle room depending on how the tests are stored, it’s best to look for a date that’s more than a few months away.

Wait to test until your period is late. We know this is hard. And we know there are tests that promise early results up to 6 days before your missed period. But even those tests — which are some of the most sensitive on the market, able to pick up lower levels of hCG — are most accurate a day or two after your missed period. In fact, if you read the fine print, you’ll likely see that advertised claims of 99 percent accuracy only apply to this time frame.

Use first morning pee to test. Your urine usually has the highest concentration of hCG at this time.

Discard test strips after the time limit. Most brands say to read your test results within 5 to 10 minutes. The problem with digging a test out of the trash later — and don’t be ashamed if you’ve done this, as many women have — is that an extra line may have appeared simply due to moisture or evaporation and not due to pregnancy.

Remember that a faint line is still a positive. However, our eyes can play tricks on us — so if the line is so faint that it has you second guessing yourself, test again in a couple days.

Store unused tests is a cool, dry place. This might not be your bathroom. Although unlikely if individually wrapped strips remain sealed, moisture can make tests less accurate.

If you have pregnancy tests that have expired, it’s best to toss them. And regardless of what type of home pregnancy test you use, always confirm pregnancy with your healthcare provider. They’ll give you an hCG blood test, which is the most accurate way to test for pregnancy and begin your prenatal screening.

You wouldn’t drink from a carton of milk that expired weeks or months ago. Using an expired pregnancy test may not have the same physical risks, but getting a false positive or false negative can affect your mental and emotional health — so it’s best to avoid the risk.