A home pregnancy test detects human chorionic gonadotropin (hCG), the pregnancy hormone. Although there are many different types and brands of home pregnancy tests, they all function in basically the same manner. These tests will check the urine to try to detect the presence of the hcG hormone.
Available Home Pregnancy Tests:There are several varieties of home pregnancy tests. In general, they all operate in a similar way. Most pregnancy tests will use a dipstick to collect and analyze the urine. These one-step kits are usually considered the most convenient to use; most sticks can be briefly held in the urine stream or dipped into a collection cup.
There are also other home pregnancy tests which require a woman to mix a small amount of her urine with a special liquid or powder. Even though each test may function in the same manner, it is still important to read the test’s instructions as these could vary between each pregnancy test brand.
How Home Pregnancy Tests Work:
These tests measure the amount of the pregnancy hormone, hCG, found in a woman’s urine. The female body will only release hCG when she is pregnant (when a fertilized egg implants in the uterus). In most women (but not all), this occurs about 6 days after conception. The hCG levels increase with each passing day of a pregnancy, doubling about every 2 days.
Home pregnancy tests can reliably detect this hormone approximately one week after a missed period. Although some home tests could detect hCG as early as a missed period, the majority of these are not sensitive enough to guarantee the results if taken this soon.
Accuracy of a Pregnancy Test:Accuracy claims can be somewhat misleading. Many home pregnancy tests typically maintain a 99 percent accuracy rate or better. The problem lies in the fact that these tests also imply that this accuracy could be expected if you take the test as early as the day of a missed period.
A 2004 study published in The American Journal of Obstetrics and Gynecology by Dr. Laurence Cole and researchers at the University of New Mexico confirmed the deceptive claims of many of these early pregnancy tests. Cole explained that these claims are misleading due to the high degree of variation in the amount of hCG occuring in the urine on any given day after implantation occurs. Researchers found that for early detection of pregnancy, most tests were not sensitive enough to detect hCG on the first or second day after a missed period.
In fact, out of the 18 brands tested, only one, the First Response, Early Result Test, was sensitive enough to "consistently detect 12.5 mIU (milli-International Units per milliliter of urine) of hCG, considering the manufacturer's suggested read time, and produced both clear and faintly discernible positive results" on the first and second day after a missed period. This level of sensitivity (12.5 mIU) is required to detect 95 percent of pregnancies at the time of a missed period.
According to Dr. Cole, et al., "three brands gave some form of a positive result, whether clear or faintly discernible at the suggested reading time using the 25 mIU concentration of hCG (Clear Blue Easy, One Minute; Clear Plan Easy; First Response, Early Results)." This level of sensitivity is able to detect 80 percent of pregnancies on the first or second day of a missed period. Most of the other tests were only able to detect hCG in 16 percent of pregnancies when tested a day or two after a missed period.
Determining the Sensitivity of a Home Pregnancy Test:Usually, the more sensitive the test, the earlier you can get an accurate pregnancy test result. Consumer Reports experts advise when buying a home pregnancy test, "the most sensitive tests currently detect about 15 to 25 mIU of hCG, corresponding to detection of pregnancy within about a day of a missed period for 90% of women."
For most women, the more sensitive the test is, the more accurate the result; however, if a woman has hCG in her system from a recent birth, miscarriage or fertility drugs, a less-sensitive test may be a better option.
When trying to determine how sensitive a test is, you can check the package insert. Most should explain the lowest mIU concentration of hCG that the test can detect. In theory, a pregnancy test that maintains that it could identify hCG at 25 mIU should be more sensitive than one that can identify this hormone at 40 mIU. The only thing to be aware of is that a woman produces different kinds of hCG during a pregnancy, so sometimes the sensitivity claims of pregnancy tests do not actually indicate that the test will pick up on the type of hCG most associated with an early pregnancy.
The Reason Why Early Home Pregnancy Tests Can Make These False Claims:
These misleading claims tend to be somewhat of an advertising gimmick. The FDA regulates that a home pregnancy test can maintain greater than 99 percent accuracy as long as the manufacturer shows that at least 99 percent of the time, in a lab, their test functions as well an existing test. The home pregnancy tests currently available are, in fact, more sensitive than earlier tests, so the companies can maintain these claims.
The "catch" is that these manufacturers make the accuracy claims in a general sense; they then suggest (separately) that a woman could use the test as early as the day of a missed period. However, the lab results typically do not reflect the test's ability to detect pregnancy this early on.
When to Take a Home Pregnancy Test:
It is important to point out that the 90-99 percent test accuracy claims are generally true once a woman is further along in a pregnancy – just not during the first few days. This is why it is usually best to wait at least one week after a missed period to take a pregnancy test. Keep in mind, that even though many home pregnancy tests can be taken as early as the first day of a missed period (and claim to be 99 percent effective on the day of a missed period), most of these pregnancy tests will not consistently detect a pregnancy this early.