Since their invention in 1976, home pregnancy tests have found their way into most women’s bathroom cabinets at some point in their lives .
Whether you’re trying to get pregnant, trying not to get pregnant, or simply open to the possibility, knowing if, when and how you could find yourself flirting with motherhood is a pretty major thing.
Home pregnancy tests are, in so many ways, a tool that helps you plan the next chapter of your life, and are an important invention in herstory. But like with any medical invention, it’s important to ask questions too so that you can use your home pregnancy test in an informed and empowered way.
Here, we’re going to have a look at how home pregnancy tests work, how clear-cut they are, and if (and when) they might not be providing you with the most accurate reading.
How do home pregnancy tests work?
If you’ve never used one, you’ve probably still seen the broad strokes in movies or TV shows — a person pees on a little test strip on a stick, and after a few minutes, a line or a plus sign or a smiley face appears, generally followed by elation or tears or a moment of panic.
All in all, the peeing on a stick thing isn’t inaccurate. Home pregnancy tests do use urine to work out if you’re pregnant or not, although most recommend peeing in a cup and dipping the stick in, as opposed to peeing directly on the stick's test strips themselves.
But how does that confirm pregnancy?
About 8 days after you conceive, your body starts to produce a hormone called human chorionic gonadotropin, or hCG for short. This hormone is responsible for developing the placenta, and during the first 12 weeks of pregnancy it spikes pretty drastically — it’s got a lot to do after all.
Most studies indicate that it increases at a rate of about 50% per day during that first trimester, reaching a peak of about 100,000 mlU/ml before dipping at the start of your second trimester to about 20,000 mlU/ml and remaining stable at that number for the rest of your pregnancy .
The hormone hCG is detectable in both your blood and your urine, and it’s hCG that both home urine tests and blood pregnancy tests are looking for. The test itself is in-built with monoclonal or polyclonal antibodies which detect hCG and from that, can tell you if you’re pregnant or not pregnant .
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Do pregnancy tests have an expiration date?
The short answer is yes, pregnancy tests do have an expiration date, although how far away that is from when you purchased it really depends on the brand and make of the kit.
Generally speaking though, pregnancy tests have a shelf life of 2-3 years, so hopefully too many won’t be going to waste .
Do unused pregnancy tests expire?
Does the bottle of milk in your fridge expire if you don’t open it? While keeping a pregnancy test sealed for as long as possible can slow its ageing down, ultimately the expiry date is the expiry date, and so your home pregnancy test will lose accuracy as the product degrades.
Do expired pregnancy tests work?
They may do! Unfortunately, though, the results can’t be trusted as they can result in a false positive or a false negative.
For any person trying to get pregnant (or those not), this can cause a lot of emotional stress, so ensuring your results are as accurate as possible by ensuring you're not using an expired test can be really important.
Why do pregnancy tests expire?
That antibody that detects the hCG hormone in the urine has a lifespan. It can’t live forever on the test strips, meaning that it will start to break down over time, in turn causing inaccurate results.
The expiration date printed on the box should be looked at seriously, as it’ll have been printed with the lifecycle of the antibody in mind .
Are there other reasons a pregnancy test result might be wrong?
Many studies indicate that home pregnancy tests have a near 100% accuracy rate of detecting hCG. That said, they don’t always have a 100% accuracy rate when it comes to detecting pregnancy, and they can sometimes give you what’s known as a false positive or a false negative .
There are a lot of reasons for this, from environmental factors to things that may be happening in your body, to issues with the test (such as an expired test), to simple things like user error.
Interestingly, false positives, meaning your home pregnancy test says you’re pregnant when you’re not, are more common than false negatives, meaning your test says you’re not pregnant when you are.
What causes a false positive pregnancy test?
False positive pregnancy test results really have the most variables when it comes to home tests, and are often caused by women using tests that are past their expiration date.
That said, there are many other causes for a false positive reading, including:
- Contaminated urine collecting cup. The most common cause of this is that there may still be detergent residue on the cup you’re peeing into. Making sure that the container is clean and dry before you urinate into it is crucial.
- A faulty test kit. As with any product, you might sometimes get a dud. Purchasing 2 can help to reduce the risk of receiving an incorrect result.
- A recent birth or miscarriage. Your body’s hormones take a while to resolve themselves after birth or a miscarriage and understanding that that could affect your home pregnancy test is really important. If this is the case, it's a good idea to speak to your doctor or women's healthcare provider.
- An ovarian tumour or some other type of hCG-producing growth .
What causes a false negative pregnancy test?
While rarer than a false positive pregnancy test, false negatives do happen as well, and being aware of why and how that can happen can help you to guide your expectations and actions.
After an expired test, the main causes of a false negative test are:
- User error, in particular not following the instructions for each unique home pregnancy test kit correctly, or testing too soon after the pregnancy began. It takes 8 days after conception for your body to start producing hCG, and even then, it takes a while to rise to detectable levels . If you’re unsure, leave it for another couple of days and try again.
- Your urine may be too diluted. While it’s great to be hydrated, a potent stream of urine, particularly your first-morning urine, can cause more accurate results.
- Some medications such as antihistamines can also impact your urine in a way that conceals hCG levels. Speaking to a medical professional is always recommended.
Whether you've had a missed period, feel nauseous or fatigued, or have tender, swollen breasts, most pregnancy tests should give you an accurate result as to whether or not you're pregnant, but you should always read the instructions carefully and make sure to check the expiration date stamped on the box.
Don't forget too that while your store pregnancy tests are typically good at detecting pregnancy, if you have any doubts, you should speak to your doctor or women's healthcare provider who will conduct a blood test to remove any doubt.