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How do home pregnancy tests work?
Home pregnancy tests can tell you if you're pregnant by detecting the presence of a hormone called human chorionic gonadotropin (hCG) in your urine.
This hormone is produced by cells that will develop into the placenta. It first enters your bloodstream when the fertilized egg starts to implant in the lining of your uterus, as early as six days after fertilization.
The amount of hCG in your body then increases rapidly over the next few weeks, often doubling every two days or so. When a test detects the hormone in your urine, it will show you a positive result.
How soon can I take a home pregnancy test?
Some home pregnancy tests claim they're sensitive enough to give you a positive result as early as five days before you would expect your next period. And some women will have produced enough hCG to get a positive result at that point. So if you're anxious to know and don't mind spending the money, go ahead and try it. If you get a negative result, you can just wait and test again later if you still haven't gotten your period.
Most home pregnancy tests claim to be "greater than 99 percent accurate" if you use them on the day you miss your period, but one study found that some tests aren't sensitive enough to guarantee an accurate result at that point either.
Researchers evaluated pregnancy tests from different manufacturers and found that only one brand of test (both the digital and non-digital 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 more likely to get an accurate result if you wait a few days to a week after you expect your period before testing.
How can these tests claim to be accurate so early?
According to Food and Drug Administration (FDA) regulations, a home pregnancy test can claim to be "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. Today's home pregnancy tests are more sensitive than previous products, so 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.
How can I tell which tests are the most sensitive?
It's not easy. New products come out frequently, and manufacturers can make improvements at any time.
But some package inserts provide information about a test's sensitivity – that is, they report the lowest concentration of hCG in milli-International Units per milliliter (mIU/ml) of urine that the test can detect. For example, a pregnancy test that claims to be able to detect hCG at 20 mIU/ml theoretically should be more sensitive than one that claims to detect it at 50 mIU/ml.
How do I use a home pregnancy test?
First check the expiration date on the package, especially if you've had it 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 use the dropper provided to place a small sample in the testing well. With others, you can pee directly onto the testing 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. "Digital" tests tell you in words whether you're pregnant. Most have a control indicator (often a second line or symbol) to indicate whether the test is valid.
It may take up to 10 minutes to see results. If the control indicator doesn't show up properly, the test may be faulty. If this happens, you can usually call the manufacturer and have them send you a new one (though it probably won't arrive soon enough to use that same month).
If you have 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.
So don't assume that one negative result means you're not pregnant. The amount of hCG produced is different for every woman and varies with each pregnancy. Just because you were positive early in your first pregnancy doesn't mean you'll be positive early in your second one.
If you still haven't gotten your period (or a positive result) a week or so after you would expect it, contact your healthcare provider.
Is it possible to get a false positive result?
False positives are uncommon, but they can happen under certain circumstances:
- You had a miscarriage or a pregnancy termination in the previous eight weeks or have a molar pregnancy.
- You've taken a fertility drug containing hCG (used to induce ovulation in fertility treatments).
- You have a rare medical condition, such as an hCG-secreting tumor.
- You're using an expired or faulty test kit.
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 a fertilized egg implanted in your uterus and developed just enough to start producing hCG, but then stopped developing for some reason. This form of early miscarriage usually happens when the fertilized egg has defects that prevent it from growing normally.
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 take 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. No matter what result you get from a pregnancy test, call your healthcare provider right away if you:
- feel dizzy or faint.
- have abdominal pain (especially a sharp or stabbing pain in your abdomen or on one side of your pelvis).
- have abnormal bleeding.
How are home pregnancy tests different from the tests used in a medical office?
Most healthcare providers use a urine pregnancy test, just as you would at home. However, your provider may test your blood as well to find out your exact level of hCG or see what's happening to the level over time – to tell whether you're having a miscarriage, for example. It takes anywhere from an hour to a day to get the results of a blood test.
Where can I buy a home pregnancy test?
You can buy them at most drugstores and online. They generally cost between $6 and $20, and usually contain two or three tests. Digital tests typically cost more, and bulk packs of testing strips cost less.
Can I order a blood test online?
There are testing companies that will allow you to pay online and then go to a lab and have a blood sample taken. The testing company will provide you with your result the next day by phone or online. These tests start at about $40. They claim to be able to provide you with accurate results as early as 6 to 8 days after ovulation.
Still confused about whether you're pregnant?
Take our Am I Pregnant? quiz