How do home pregnancy tests work?
Home pregnancy tests measure the presence of a telltale hormone in your urine called human chorionic gonadotropin (hCG). This hormone, produced by cells from the placenta, first gets into your bloodstream when the fertilized egg implants in your uterus (about six days after fertilization). The amount of hCG in your body then increases rapidly over the next few weeks, doubling in amount about every two days. By 14 days after fertilization (about the time you would expect your period), some home pregnancy tests may be able to detect the hormone in your urine and give you a positive result. But in spite of their claims, most are not sensitive enough to guarantee you an accurate result at this point. In fact, you're much more likely to get an accurate result if you wait until a week after your expected period before testing.
How accurate are they on the day you miss your period?
Most home pregnancy 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 2004 in the American Journal of Obstetrics and Gynecology has shown that this is misleading. The fact is that the amount of hCG in the urine on any given day after implantation can vary a great deal from one woman to another. Laurence Cole and other researchers at the University of New Mexico evaluated 18 currently available tests and found that only one was consistently sensitive enough to detect the levels of hCG that most pregnant women were likely to have on the first day of their missed menstrual period. Most of the other tests were only sensitive enough to pick up about 16 percent of pregnancies at that point, though many were likely to be accurate a week after an expected period was due.
So how can all these tests claim to be accurate so early in pregnancy?
They don't. They only claim to be "greater than 99 percent accurate" in general, and then separately, they suggest that you can use them as early as the day you miss your period. According to the current Food and Drug Administration (FDA) regulations, in order to claim that a home pregnancy test is "greater than 99 percent accurate," the manufacturer only needs to demonstrate 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 they are able to maintain 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. In the New Mexico study, First Response Early Result was the most sensitive test the researchers checked, though the test showed only a faintly positive result for the lowest levels of the hormone. But new products come out all the time and other brands may make improvements to their products at any time. Consumer Reports ranks 18 available tests on its Web site (though you have to subscribe to see the report).
Some package inserts provide some information about a test's sensitivity — that is, they report the lowest concentration of hCG (in 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. Unfortunately, this information is often misleading because there are actually different kinds of hCG and these numbers won't necessarily tell you how good a test is at detecting the kind that's most relevant in early pregnancy.
How do I use a home pregnancy test?
First check the expiration date on the test and make sure it's still valid, 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, so it's best to throw it away and get a new one. When you're ready to test, read the directions carefully because they'll 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 a stick. (Some will let you do either.)
The tests also vary in how they display results: Some show pink or blue lines on the test strip, while others reveal a red plus or minus sign in a window. 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, though it might not arrive soon enough for you to use it that month. Most tests say that you can check results in about five minutes, although the New Mexico study found that in some cases, you may have to wait ten minutes for a positive result.
If the test shows a negative result, wait another few days or a week and try again if you still haven't gotten your period. (If you ovulated later in your cycle than you thought, you may have taken the test too early to get a positive result.) For best results, try taking the test first thing in the morning, when your urine is most concentrated.
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 be pregnant (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 practitioner so you can try to find out why.
Is it possible to get a false positive result?
False positives — when the test says you're pregnant but you're not — do happen. The New Mexico study found two brands that gave occasional false positive results. If you get an early positive result and then get your period soon after, you may also have had what's called a "chemical pregnancy." That means a fertilized egg implanted in your uterus and developed just enough to start producing hCG but then stopped developing for some reason. This happens with about 30 to 50 percent of all fertilized eggs because they're abnormal or otherwise incapable of developing into and surviving as an embryo. If this is the case, you'll go on to get your period (though it may be a little heavier and a few days later than usual). When pregnancy tests were less sensitive than they are today, these so-called "chemical pregnancies" were never identified. Many practitioners 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 can give you either a positive or negative result on a pregnancy test. Call your practitioner right away if you have any abdominal pain or abnormal bleeding, no matter what a pregnancy test tells you.
How are home pregnancy tests different from ones performed in doctors' and midwives' offices?
Many clinics use the very same kind of urine pregnancy tests that are marketed for home use. Doctors and midwives generally only use blood tests to test for pregnancy when they need to know exactly how much hCG is in your blood or what's happening to the level over time — if they think you might be miscarrying, for example.
• 10. Tender, swollen breasts
• 9. Fatigue
• 8. Implantation bleeding
• 7. Nausea or vomiting
• 6. Increased sensitivity to odors
• 5. Food aversions
• 4. Frequent urination
• 3. A missed period
• 2. Your basal body temperature stays high
• 1. The proof: A positive home pregnancy test
• Related Links
Most likely you won't notice any pregnancy symptoms until about the time you've missed a period, or a week or two later. If you're not keeping track of your menstrual cycle or if it varies widely from one month to the next, you may not be sure when to expect your period. But if you start to experience some of the symptoms below (not all women get them all) and you haven't had a period for a while, you may very well be pregnant. Take a home pregnancy test and find out for sure!
10. Tender, swollen breasts
One of the early hallmarks of pregnancy is sensitive, sore breasts caused by increasing levels of hormones. The soreness may feel like an exaggerated version of how your breasts feel before your period. Your discomfort should diminish significantly after the first trimester, as your body adjusts to the hormonal changes.
9. Fatigue
Feeling tired all of a sudden? No, make that exhausted. Increased levels of the hormone progesterone and the stress your body undergoes as it adapts to pregnancy can make you feel as if you've run a marathon when all you've done is put in a day at work. You should start to feel more energetic once you hit your second trimester, although fatigue usually returns in the third trimester.
8. Implantation bleeding
Some women have a small amount of vaginal bleeding around 11 or 12 days after conception (close to the time you might notice a missed period). The bleeding may be caused by the fertilized egg burrowing into the blood-rich lining of your uterus — a process that starts just six days after fertilization — but no one knows for sure. The bleeding is very light (appearing as red spotting or pink or reddish-brown staining) and lasts only a day or two. (Let your practitioner know if you notice any bleeding or spotting, particularly if it's accompanied by pain, since this can be a sign of an ectopic pregnancy.)
7. Nausea or vomiting
If you're like most women, morning sickness won't hit you until about a month after conception. (A lucky few escape it altogether.) But some women do start to feel queasy a bit earlier. And not just in the morning, either — pregnancy-related nausea and vomiting can be a problem morning, noon, or night. About half of women with nausea feel complete relief by the beginning of the second trimester. For most of the rest it takes another month or so for the queasiness to ease up.
6. Increased sensitivity to odors
It's not uncommon if you're newly pregnant to feel repulsed by the smell of a bologna sandwich or cup of coffee and for certain aromas to trigger your gag reflex. Though no one knows for sure, this may be a side effect of rapidly increasing amounts of estrogen in your system.
5. Food aversions
While some women insist they crave certain foods during pregnancy, food aversions are even more common. You may suddenly find that certain foods you used to enjoy are now completely repulsive to you. This feeling may come and go or last throughout your pregnancy.
4. Frequent urination
Shortly after you become pregnant, you may find yourself hurrying to the bathroom all the time. Why? Mostly because during pregnancy the amount of blood and other fluids in your body increases, which leads to extra fluid being processed by your kidneys and ending up in your bladder. This symptom may start as early as six weeks into your first trimester and continue or worsen as your pregnancy progresses and your growing baby exerts more pressure on your bladder.
3. A missed period
If you're usually pretty regular and your period doesn't arrive on time, you'll probably take a pregnancy test long before you notice any of the above symptoms. But if you're not regular or you're not keeping track of your cycle, nausea and breast tenderness and extra trips to the bathroom may signal pregnancy before you realize you didn't get your period.
2. Your basal body temperature stays high
If you've been charting your basal body temperature and you see that your temperature has stayed elevated for 18 days in a row, you're probably pregnant.
And finally...
1. The proof: A positive home pregnancy test
In spite of what you might read on the box, many home pregnancy tests are not sensitive enough to detect pregnancy until about a week after a missed period. So if you decide to take one earlier than that and get a negative result, try again in a few days. Once you've gotten a positive result, make an appointment with your practitioner.
If your period is usually right on schedule and you're now late, you might well be pregnant. Some pregnant women do notice some light spotting around the time their period is due. This bleeding may be caused by the fertilized egg burrowing into the blood-rich lining of your uterus (a process that starts just 6 days after fertilization) but no one knows for sure. And it's a lot lighter than a typical period — just a little spotting over a day or two. On the other hand, it's not uncommon to get your menses a few days late every once in a while. The best way to find out is to take a home pregnancy test.
Pregnancy tests detect the presence of human chorionic gonadotropin (hCG), a placental hormone that your body starts making more of around the time a fertilized egg implants in your uterus. These home tests are now able to detect fairly low levels of hCG. By the time your period is a few days late, you should be able to get a positive result if you're pregnant. For best results, use your first urine of the morning (which is more concentrated so it should have more hCG) and follow the directions carefully. If you get a negative result but your period still doesn't arrive in the next week, repeat the test.
In the meantime, if you spot again or if you feel other symptoms such as pelvic or abdominal pain, dizziness, or lightheadedness, call your practitioner immediately so you can be evaluated to make sure that you don't have an ectopic pregnancy. (You may have an ectopic pregnancy even if you don't get a positive result on a pregnancy test.) A small amount of bleeding or cramping may also be a sign of impending miscarriage, but some women do spot for no discernable reason in the first trimester with a normal pregnancy.
If you do get your period (instead of a positive pregnancy test) and haven't yet scheduled a preconception visit with a midwife or doctor, consider doing so now.