You’ve opened the box, ripped open the plastic, peed on the absorbent strip and impatiently awaited your results. But have you ever wondered just how much you can trust that little stick? With so many brands and styles on the market—from fancy early detection models to cheap ones you can pick up for next to nothing at the dollar store—choosing the right pregnancy test can feel overwhelming. Carrie Schram, a family doctor at Women’s College Hospital and medical director of the cycle monitoring program at Hannam Fertility Centre in Toronto, answers everything you’ve ever wondered about pregnancy test accuracy, the importance of price and how to avoid a false negative.
Any pregnancy test you can find, regardless of brand, style or price, will look for one simple thing: a detectable level of human chorionic gonadotropin (hCG), the hormone that is produced by a newly implanted embryo. In a typical pregnancy, the hormone level doubles every 48 to 72 hours and usually reaches a level that can be detected by a urine test around 12 to 14 days after conception (or a couple of days before your expected period). If a test is more sensitive, it may be able to detect lower levels of hCG (that is, an earlier test result).
If your pregnancy is in its early stages and your body has not yet produced enough hCG for the test to detect, you could get a false negative, which means you’re actually pregnant, even though the test says you’re not. The earlier you test, the higher the chances that this scenario will occur. If you get a false negative but still suspect that you’re pregnant, repeat the test in a few days. If you miss your period, test again and book an appointment with your healthcare provider if you still get a negative result. “It’s reasonable to visit your doctor for more-definitive blood work and see if there is another reason why you haven’t had a period,” says Schram.
A false positive—when a pregnancy test says you’re pregnant but you aren’t—is much rarer, but it can occur. The medication Ovidrel (choriogonadotropin alfa), which is a form of the hormone hCG that is used in fertility treatments, can be detected by a pregnancy test up to 12 days after it’s injected. If you have a tumour that secretes hCG, your pregnancy test could also show a false positive, but this is very rare. In addition, pregnancy tests can pick up on pregnancies that can’t be carried to term, such as ectopic pregnancies (most often in the fallopian tubes) and very early miscarriages.
The earlier you test, the more likely you will get a false negative. “Implantation usually occurs about nine days after ovulation, and periods usually start about 14 days after ovulation,” says Schram. That means that if you test too soon before your expected period, you will get a negative test result, even if you are pregnant. From a results perspective, the best time to test is the day of your expected period or afterwards.
Some specialty tests are able to detect a pregnancy as early as six days before a missed period (which, FYI, is not the day your period is expected but the day after). “But the earlier you test, the less accurate the test will be because ovulation isn’t perfectly consistent from month to month and implantation can vary by two or three days post-ovulation,” she says. “I have seen patients get quite upset when an early test is negative, only to find totally appropriate hCG levels present the day their period is due.”
If your periods are irregular and vary by more than a couple of days each month, it will be harder to know when to test for pregnancy, but results from ovulation tracking can act as a guide. If your urinary ovulation monitor, which detects luteinizing hormone (LH)—the hormone that triggers ovulation—gets a positive result, you can test for pregnancy about 10 to 14 days afterwards. (Note: This method won’t work if you tend to get false-positive urinary LH readings.) The same is true if you track your temperature and notice the subtle rise that occurs after ovulation. Simply wait 10 to 14 days before taking a pregnancy test. “If you don’t want to track ovulation or find it difficult, test for pregnancy once a week,” says Schram.
Pregnancy test accuracy is not affected by price. If your dollar store pregnancy test says you’re pregnant, you almost certainly are. That said, more expensive tests may have increased sensitivity, so they’ll be able to detect lower levels of hCG and provide positive test results at an earlier date than cheaper tests. “That’s why you shouldn’t rely on dollar store tests to test really early,” says Schram.
Any test you find at a Canadian drugstore, grocery store or dollar store will have more than 99 percent accuracy if you test on the day of your expected period or afterwards. More-expensive early detection tests may include a table with accuracy percentages for up to six days before a missed period, and you’ll notice that accuracy decreases the earlier you try to test. (For example, the Clearblue Early Detection Pregnancy Test claims more than 99 percent accuracy if you test the day of your expected period or one, two or three days before; 96 percent accuracy four days before; and 79 percent five days before. These numbers are comparable to the First Response Early Result Pregnancy Test.)
One of the most valuable pieces of information you can have is the hCG level that the test can detect. The lower the amount of hCG required, the more sensitive the test and the earlier you’ll be able to confirm a pregnancy. Unfortunately, this information can be hard to come by. Clearblue is transparent and posts these levels on its website. Other brands list them on the directions inside their packages (so you won’t know the detectable hCG level unless you buy a test). Other brands refuse to give out this information at all.
Here are the detectable hCG levels found in some of the most common pregnancy tests in Canada. They’re almost all the same, regardless of price or promised “early result.” Remember, the lower the number, the more sensitive the test.
Clearblue Early Detection Pregnancy Test: 10mIU/mL
Clearblue Pregnancy Test with Weeks Indicator: 25mIU/mL
Clearblue Rapid Detection Pregnancy Test: 25mIU/mL
Life Brand One Step Pregnancy Test (house brand of Shoppers Drug Mart): 25mIU/mL
Life Brand Early Result Pregnancy Test (house brand of Shoppers Drug Mart): 25mIU/mL
Medi Care One Step Pregnancy Test (sold at Dollarama): 25mIU/mL
Equate One Step Pregnancy Test (house brand of Walmart): 25mIU/mL
“For most women, it’s actually faster to grab a test from a pharmacy or dollar store,” says Schram. “The first step with us is also a urine test, which tests hCG at the same concentrations as the test you use at home. Plus, most offices don’t stock fancy early detection tests, so ours may actually miss very early pregnancies that you could pick up at home. When medically indicated, we can also order serum (blood) pregnancy tests, but these can take up to 24 hours to come back.”
1. Check the expiry date: Using an expired test could result in a false negative.
2. Take the test first thing in the morning: All pregnancy tests check hormone levels in urine. Taking the test after you’ve had a lot of water or fluids will dilute your urine, increasing the chances of a false negative test, especially for an early pregnancy. It’s best to take the test after you’ve gone several hours without drinking.
3. Follow the directions: Since instructions vary by brand, make sure that you take a quick read. For example, a package might say that you should check your results within 10 minutes, so follow suit. “Leaving the test out too long can lead to a ‘ghost line,’ which appears when other substances in your urine deposit on the detection line as the urine evaporates,” says Schram. The false line will affect the accuracy of your reading.
The takeaway? Pregnancy test accuracy is high, and at-home tests, regardless of style or price, are excellent at detection when there is enough hCG in your urine—usually 12 to 14 days after conception. If your test result is positive, you are almost certainly pregnant. But if your test result is negative, the pregnancy may be too early to be detected. Simply test again if you miss your period.