Home pregnancy tests should really be called human chorionic gonadotropin (hCG) tests, said David Grenache, a clinical chemist who specializes in diagnostic testing. The tests can detect the presence of hCG, a hormone that’s measurable in urine after a fertilized egg burrows into the uterine wall. But they don’t truly tell you that you’re carrying a viable pregnancy. There are also rare medical conditions or medical treatments that make hCG detectable in the urine. So even though it’s a good indicator for pregnancy, a positive result doesn’t always mean you’re pregnant.
This burrowing event typically happens anywhere from six to 12 days after ovulation, when a fertilized egg first leaves the ovary. Ovulation occurs between days 12 and 16 of the menstrual cycle, which begins on the first day of a person’s period. Health-care providers measure pregnancy from day 1 of the cycle, which is the first day of a person’s last period. So by the time hCG is present at measurable levels (around day 18 to 28), from a medical standpoint, a pregnancy may be three or four weeks along, even though the fertilized egg has only just implanted.
Once the body begins producing more hCG, its amount in the blood and urine roughly doubles every two or three days for the first eight to nine weeks of pregnancy. After implantation, someone might have anywhere from 5 to 50 mIU/mL of hCG in their pee (mlU/mL means milli-International Units per milliliter, a standardized unit).
Test accuracy and sensitivity
Some researchers have estimated that 25 mIU/mL is the lowest amount of hCG a test must be able to measure in order to identify 99% of true positives on the day of an expected period or later. This measure of how frequently a test correctly identifies a positive or negative is a test’s accuracy.
Accuracy changes depending on what day you use the test. A test that claims to be 99% accurate on the day of an expected period may claim just 50% accuracy a few days earlier. Pretty much every pregnancy test has a chart on the box or in the instructions outlining these numbers. You may notice that many tests display their claimed accuracies relative to the day of a “missed period,” which can be misleading. What this actually means is the day after your expected period.
Of course, this is all assuming you have an average-length cycle and that you always ovulate exactly two weeks after your period. In reality, test accuracies are affected by the lengths of different phases of the menstrual cycle, when an egg may have been fertilized, and how long it took a fertilized egg to reach the uterus, among other things.
A test’s accuracy is different from its sensitivity, which is defined as the lowest amount of hCG a test can detect 99% of the time. For some tests, this value can be as low as 10 mIU/mL, but for most it is around 25 mIU/mL. Manufacturers determine this using standardized samples with known concentrations of purified hCG. Some at-home tests, like the First Response Early Result test we recommend, can detect hCG levels as low as 6 mIU/mL, but only about half the time. The sensitivity of the same test can also vary from person to person because everyone has a different mixture of hCG forms in their urine.
All pregnancy tests detect whole hCG, which has an alpha and a beta region. As hCG degrades, other varieties appear in the urine, including the alpha and beta region on their own and a variant called the beta core fragment. There’s also hyperglycosylated hCG (hCG-H). Some research has suggested that detecting hCG-H could allow a pregnancy test to work earlier. But, Grenache explained, by the time hCG-H appears in urine, regular hCG is already present at measurable levels. Some tests detect several of these variants, but it’s not clear whether that makes them more sensitive.
False positive and false negative results
It’s possible for any pregnancy test to give false positive or false negative results.
False positives are rare. But they may occur if, for example, you test early, when hCG levels are high enough to be picked up by a pregnancy test but there’s no viable pregnancy. This could be due to a chemical pregnancy, when the fertilized egg fails to continue growing after implanting or never finishes implanting. Often, this will end in miscarriage around the time of an expected period.
Tests with very high sensitivities are more likely to detect chemical pregnancies, which Stanford fertility specialist Bavan said could cause false hope and then letdown for people who are trying to conceive, as well as unnecessary stress for people hoping for a negative result.
Super-high sensitivity doesn’t offer much if you’re looking for reassurance that you are not pregnant. Bavan said that regardless of the sensitivity of the test, if the results are negative, you’ll still want to test again a week after your expected period to be totally sure. At that point, any test will be sensitive enough for most people. If you have a history of irregular periods, pregnancy symptoms, pain, or abnormal bleeding, she recommended following up with a medical provider right away.
There are other rare situations where a more sensitive test could be more likely to give false positive results. For example, hCG can increase during perimenopause. One study found that 1.3% of home pregnancy tests taken by women ages 41 to 55 would be false positives. Many manufacturers measure the false positive rates of their pregnancy tests using urine samples from women in this age range. When making our ratings, we considered whether this extra step was taken.
False negatives—where a test reads negative but a potentially viable pregnancy has begun—are more common. Usually, these are not “false negatives” at all—they’re simply the result of a test taken when the level of urine hCG is lower than what the test can detect. “I do advise patients that you ideally want to check on the day of your missed period onward for the most accurate results,” Bavan said.
It is also possible to get a false negative result after using a test on a day the box says it should be 99% accurate. This is because of how different the levels of urine hCG are from person to person, and even how much they can change throughout the day in the same person. Your hCG may be on the low end, or your pee may be very dilute.
You can also get a false negative if your urine hCG levels are very high. Sometimes people test for days or weeks after a confirmed pregnancy for reassurance that they are still pregnant. Grenache warns that this may result in false negatives due to something called the hook effect, when an excess amount of hCG makes it difficult for the test to work properly. A similar problem called the variant hook effect can also occur, usually starting by around nine weeks’ gestation, when one variety of the beta fragment of hCG is present at higher levels than whole hCG. “Paradoxically, you end up with a less positive, or sometimes a negative, result if you test too late in pregnancy,” he said.
How home pregnancy tests work
So how do the tests actually detect hCG? Most of the action takes place along a narrow strip of a special absorbent type of paper. Each strip is pre-loaded with molecules needed to detect hCG and create a colored band and a control band. As urine containing hCG gets wicked up the paper, it passes areas where those molecules have been deposited.
Each test’s design is slightly different, but it generally goes like this: In the first area of the paper the urine hits, dyed antibodies cling to the beta region of hCG, and together the two molecules flow forward to a second area, where antibodies that cling to hCG’s alpha region are stuck to the paper. These capture the hCG and the dyed antibody, and as the dyed antibodies accumulate, they create a visible line. In the meantime, extra dyed antibody that has not stuck onto any hCG flows on to the control zone, where it gets trapped by a third antibody. It accumulates and creates a visible control line, letting the tester know they used enough urine to make the test work.
In wand tests, all of this happens inside a plastic casing with a handle, and the test zone and control zone are usually under a clear layer of plastic. The urine is picked up by an absorbent material at one end of the wand, either by the tester peeing directly on it (the “stream” method) or dipping it into a cup of urine. The absorbent tip carries the urine to the paper strip inside the wand.
Digital tests work the same way, except they have a sensor that detects the presence of the test line and control line and a screen that displays the results.
Some tests come inside a cassette instead of a wand, much like some at-home COVID-19 tests. The cassette has the same kind of strip inside, but there’s no plastic layer covering the test zone. Instead of an absorbent tip, they have a circular area where the strip is exposed. They come with a dropper you use to transfer a few drops of urine onto the exposed area. You usually have to provide your own cup.
Finally, there are just strips, with no plastic casing at all. They must be dipped into a clean cup of urine. Because they’re exposed, they may be more likely to fail after being in very high or very low temperatures or high humidity.
Any home pregnancy test will have limitations, Bavan said, so be sure to follow up with a doctor if you get a positive result, or if you have any other questions about testing for potential pregnancy at home. Greene said a blood test is the most accurate and sensitive test for hCG. According to Greene, physicians don’t always automatically order blood tests, though, so if you really need to know very early on, ask for one.
