Over-the-counter test kits have revolutionized the way women learn that they are pregnant. Because they are relatively inexpensive, easy to use, and reliable; they have brought the diagnosis of pregnancy from the doctor's office into the home. However, they aren't without limitations. If you're planning to use one it's important to know what you can - and can't - expect it to tell you.
How they work
Like earlier tests, the current generation register the presence of human chorionic gonadotropin (hCG), a hormone made by the layer of cells surrounding the developing embryo. This hormone is first produced by embryonic cells called the chorion about 7 days after conception, when the embryo becomes implanted in the uterine wall. At that time, these cells, which will eventually form the placenta, begin to secrete hCG as a chemical signal to the body that conception has occurred. This signal tells the corpus luteum, an ovarian structure from which the egg was released, to continue making progesterone and estrogen until the placenta can make them in quantities sufficient to support the pregnancy. In a normal pregnancy, hCG concentrations in blood and urine double about every two days initially, rise rapidly to a peak at 8-10 weeks, and begin to fall thereafter.
Since hCG has a strong chemical similarity to luteinizing hormone (LH), follicle stimulating hormone (FSH), and thyrotropin stimulating hormone (TSH). the tests are designed to detect the regions of the hCG molecule that are unique to that hormone. Thus, they avoid a problem common to earlier tests - mistaking LH, which is present in high levels during some phases of the menstrual cycle and at menopause - for hCG.
Home pregnancy tests are qualitative rather than quantitative; they detect the presence of hCG in the blood but not the amount of hCG present. Some home tests, such as First Response, Clear Blue Easy, and EPT are sensitive enough to detect hCG at levels as low as 25-50 International Units per liter (IU/L) of urine - an amount that many women produce as early as 3 or 4 days after implantation (10 or 11 days after conception), or before they have missed a period.
When to test
Theoretically, some home pregnancy tests are able to detect pregnancy even before the first missed period. Although knowing one is pregnant may make it possible to make recommended lifestyle changes, such as avoiding alcohol, very early in pregnancy, most practitioners don't recommend testing until the first day of an expected period for two principal reasons. First, hCG concentrations vary among women at the same stage of pregnancy, with some not reaching detectable levels until after the day of the expected period. Thus, testing too early in pregnancy can produce a "false negative" result - the indication that a woman isn't pregnant when she is. Second, a high proportion of pregnancies end in miscarriage very early, often before the first missed period, but hCG concentrations may remain above the detectable threshold for several days afterward., giving a "false positive" result - the indication that a woman is pregnant when she actually isn't.
By the first day of a missed period - what would ordinarily be the beginning of a new cycle - most currently marketed tests are reliable. They will yield a positive result for 98% of pregnant women. A negative result a week after a skipped period virtually guarantees that one isn't pregnant.
False positives have been known to occur for reasons other than early miscarriage. Women undergoing hCG treatment (A.P.L, Pregnyl, Profasi, Pergonal) to stimulate ovulation may also receive false positive results if they test within 10 days of their last injection. Occasionally a home pregnancy test may indicate that a woman is pregnant when in fact her missed periods were due to menopause. That is because around the time of menopause women have somewhate elevated levels of circulating hCG - usually between 5 and 25 IU/L. Overall, false positive rates for most tests are no higher than 2%.
False negative results are occasionally likely to occur if the urine collected has been left sitting too long -usually 30 minutes or more - before the test is performed. On rare occasions, drinking large amounts of liquid shortly before testing can increase urine volume so that the hCG is too dilute to measure.
What home pregnancy tests can't tell you
Under certain conditions your clinician may want to know the amount of hCG you're producing and whether it is doubling every 2 days as an indication of normal fetal development. If your doctor suspects you have miscarried or that you have an ectopic (extrauterine) pregnancy, trophoblastic disease, or certain cancers, or if you're certain you're pregnant but continue to receive a negative result on a urine test, he or she may ask you to take a quantitative blood test. This test is designed to detect only the portion of the molecule that is unique to hCG and is capable of measuring hCG levels within a few days of conception. Repeating the test 48 hours later, if necessary, can determine whether hCG levels have doubled.