Home pregnancy tests measure the presence of a pregnancy hormone called human chorionic gonadotropin (hCG) in your urine.
This hormone, produced by cells from the placenta, first enters your bloodstream after the fertilized egg (the embryo) implants in the lining of your uterus, shortly after fertilization. The amount of hCG in your body then increases rapidly over the next few weeks, often doubling every two days or so.
Most tests claim to be "greater than 99 percent accurate" and imply that you can use them as early as the day you miss your period, but a study published in 2011 in the journal Clinical Chemistry and Laboratory Medicine showed that this is misleading. Some tests may be able to detect the hormone in your urine at that point and give you a positive result, but most aren't sensitive enough to guarantee you an accurate result.
Researchers at the University of New Mexico evaluated both digital and manual tests from three different manufacturers and found that only one brand of test (both the digital and manual version) was 97 percent accurate in detecting pregnancy on the first day of a missed menstrual period. (The amount of hCG in the urine at this time can vary a great deal from one woman to another.) The other brands correctly predicted pregnancy about half the time.
The bottom line: You're much more likely to get an accurate result if you wait a week after your expected period before testing.
Read more about when to test to see whether you're pregnant.
According to Food and Drug Administration (FDA) regulations, a home pregnancy test can be called "greater than 99 percent accurate" if the manufacturer simply demonstrates that the test performs as well in the lab as an existing test more than 99 percent of the time. Since today's home pregnancy tests are more sensitive than previous products, it's not surprising that manufacturers are able to make this claim, but it has nothing to do with a test's ability to detect pregnancy at the time of a missed period.
It's not easy. New products come out frequently, and brands may make improvements to their products at any time.
But some package inserts provide information about a test's sensitivity – that is, they report the lowest concentration of hCG (in mIU/ml, or milli-International Units per milliliter of urine) that the test can detect. For example, a pregnancy test that claims to be able to detect hCG at 20 mIU/ml should theoretically be more sensitive than one that claims to detect it at 50 mIU/ml.
First check the expiration date on the package and make sure it hasn't expired, especially if you've had it around for a while. If you've been storing the test anywhere that gets moist or warm, like the bathroom, it may have deteriorated. If that's the case, it's better to throw it away and get a new one.
For best results, try testing first thing in the morning, when your urine is most concentrated. Read the directions carefully because they vary with different brands. Some require you to urinate in a cup and then, using a supplied dropper, place a small sample in a testing well. Others let you pee directly onto the test device. And some will let you do either.
The tests also vary in how they display the results: For example, some show pink or blue lines on the test strip, while others reveal a red plus or minus sign in a window. Many tests now have digital readouts that tell you in words whether you are pregnant or not. Most have a control indicator (often a second line or symbol) that's supposed to indicate whether the test is valid.
If the control indicator doesn't show up properly, the test may be faulty. If this happens to you, you can usually call the manufacturer and have them send you a new one, although it might not arrive soon enough for you to use that month.
If you have any questions about how to use a test, call the manufacturer's toll-free number on the package instructions.
If the test shows a negative or a faintly positive result, wait another few days or a week and try again if you still haven't gotten your period. One possibility is that you ovulated later in your cycle than you thought and took the test too early to get a positive result.
Whatever you do, don't assume that one negative result means you're not pregnant. If you don't get your period as expected, remember that you might still have conceived. (It's no time to go off on a drinking binge or do other things that are unsafe in pregnancy.) If you still haven't gotten your period or a positive result in another week or so, make an appointment with your healthcare provider to find out why.
False positives are uncommon, but they do happen under certain circumstances, such as these:
If you have an early positive result and then get your period soon after, you may have had what's sometimes called a chemical pregnancy. This means that a fertilized egg implanted in your uterus and developed just enough to start producing hCG, and then stopped developing for some reason. This form of early miscarriage usually happens when the fertilized egg is abnormal and incapable of developing into an embryo.
After a chemical pregnancy, your period may be a little heavier and a few days later than usual. When pregnancy tests were less sensitive than they are today, these very early losses were never identified. Some healthcare providers think that's another good reason to wait until a week after your period is due to perform a home pregnancy test.
Note: An ectopic pregnancy usually results in a positive pregnancy test, but it occasionally is negative because of lower hCG hormone levels. Call your healthcare provider right away if you feel dizzy or faint, have abdominal pain (particularly a sharp or stabbing pain in your abdomen or on one side of your pelvis), or abnormal bleeding, no matter what a pregnancy test tells you.
Often there's no difference. Many healthcare providers use a urine pregnancy test, just as you would at home.
Blood tests for pregnancy are usually only used to find out exactly how much hCG is in your blood or what's happening to the level over time – to see whether you're miscarrying, for example.
You can buy one without a prescription at most drugstores and online. They generally cost between $8 and $20, and most packages contain two tests.