All About Home Pregnancy Tests
Brittany Peats

How do at home pregnancy tests work?

At home pregnancy tests are an easy, cheap, private, and relatively accurate way to determine as soon as possible if you are pregnant. A week after your missed period, you can test your urine for the presence of a hormone which will indicate that you are pregnant. The hormone, called human chorionic gonadotropin, or hCG, is produced by the developing placenta soon after an embryo attaches itself to the uterine lining. This implantation takes place about six days or so after conception. Following implantation the amount of hCG increases exponentially everyday.

The hCG hormone travels through your body and will appear in urine samples which you can then test at home to determine if you are pregnant. HCG is measured in thousandths of International Units, or mIU; the most sensitive tests can detect hCG levels as low as 15 to 25 mIU while others require levels as high as 50 or even 100 mIU. If you are pregnant and the amount of hCG in your urine reaches the specified sensitivity level of a given test brand, a positive result will be indicated.

A positive pregnancy test will display a "control line" and a "test line". The control tells you that you have performed the test correctly and it is functioning properly. The test line tells you if you are pregnant or not.

If you do not wish to take an at home pregnancy test or want to double-check, your doctor can easily give you a pregnancy test. Doctors typically use a blood test to check for hCG.

How soon can I accurately take a pregnancy test?

Though many of the instructions that accompany the pregnancy tests say that you can take the test as early as the day after a missed period, this often leads to false negative results. For many women, implantation does not happen until many days later. Consequently, hCG is not apparent. Experts say women should wait 5-10 days after their period is officially late before taking the tests to ensure accuracy. Not all pregnancy tests are identical and some are more sensitive than others; the smaller the amount of hCG the test measures, the earlier it can detect whether you are pregnant. It is advisable to wait a few days and take another test. 

How can I make sure the results are as accurate as possible?

Here are some tips to follow when taking an at-home pregnancy test.

• Use your first morning urine when possible (when hCG levels are most easily detected). If not, make sure your urine had been in your bladder for at least 4 hours.
• Do not drink excessive amounts of fluids before the test to increase the volume of urine. This could dilute the hCG levels.
• Read the directions included in the test thoroughly before starting the test and follow every step precisely.
• Some fertility drugs or other medications may interfere with the test results. Check the package labeling for information.
• Adhere to the test reaction time! Every test has a time interval that must be respected, typically between five and ten minutes. Any result determined after the reaction time should be disregarded. A test will become increasingly more sensitive over time and may indicate a very faint test line based on naturally occuring levels of hCG (present in non-pregnant women and even men). Sometimes the chemical composition of urine will cause a ghost line or evaporation line. Typically colorless, an evaporation line will only appear well after the given reaction time of the test.

If I got a positive result is it possible that I am not pregnant?

Though rare, this is possible if your hormones have been altered or if there is a complication with the pregnancy. If you have recently given birth or had a miscarriage, you may have elevated hormone levels which would demonstrate a false positive. A woman who has been given an hCG injection as part of infertility treatment will test positive on pregnancy tests, regardless of her actual pregnancy status. Common infertility drugs like clomid do not contain the hCG hormone.

Some diseases may cause a false positive pregnancy test: choriocarcinomas, IgA deficiencies, heterophile antibodies, enterocystoplasties, gestational trophoblastic diseases (GTD), gestational trophoblastic neoplasms (GTN), testicular germ cell malignancies, and even other forms of cancer, e.g. lung cancer.

In addition, if there is a problem with the pregnancy, such as an ectopic pregnancy (when the fertilized egg settles in the fallopian tube instead of the uterus), hCG levels may be low and non-detectable by an at-home test.

A “chemical pregnancy” is another reason that you may see a positive result but not be pregnant. A chemical pregnancy occurs when implantation takes place - but is followed by a miscarriage. In most cases, this happens before other pregnancy signs can be noticed.

If I got a negative result, is it possible that I am pregnant?

Yes; for this reason it is advisable that you retake the test after a few days, as the accuracy of the test will increase with each passing day. Each woman’s pregnancy is slightly different: every woman ovulates at different times in her menstrual cycle and embryos implant in the uterus at different times so some women may have to wait longer for the tests to be accurate.

What should I look for when purchasing a home pregnancy test?

Most people look for four things in an at home pregnancy test: it is accurate, it can detect low levels of hCG, the results are easy to read, it is affordable, and the package contains more than one test.

There are three general types of home pregnancy tests: sticks -- which come with traditional or digital displays -- or strips. Typically you hold the handle and simply point the tip of the test into your stream of urine as you pee. If you don’t mind getting a bit more hands on, you can use strips (also know as dip strips or aimsticks). To use it, you collect urine in a cup and dip the end of the strip into the collected sample. Test strips are the most affordable option and are the same style of tests you will find at a doctor's office, clinic, or university. Both methods are equally reliable.

Interpreting the results should be pretty straight –forward for any home pregnancy test whether traditional or digital. The traditional sticks display colored lines or figures while the digital tests may show words. Though the digital readout may be appealing to some, the downside is that they are expensive and are less sensitive than more traditional tests. Interpretation is never foolproof however, even with digital kits.

If you think you may need a second test a double package is generally more economical than paying separately for a second kit. It is suggested that all women take more than one test for increased accuracy, especially if your periods are typically irregular or you're testing very soon after a missed period. For brand-name tests, which are found in drugstores, expect to spend between 5 and 30 dollars. Unbranded tests are more affordable, and equally reliable, FDA-approved, and typically more sensitive than drugstore tests. Digital kits cost a bit more.

The most sensitive tests currently detect about 15 to 25 mIU of hCG, which means that for 90% of women, the tests could detect if they are pregnant about a day after they miss a period. (Experts suggest that women wait longer to use the tests for increased accuracy.) Using a test which can detect such a low amount of the hormone is advantageous because you can find out sooner if you are pregnant. However, if you already have low levels of have hCG in your system from a miscarriage, recent birth or some fertility drugs, may want to consider a less-sensitive test.

All of the tests discussed below are very accurate; indeed, they exhibit over 99% accuracy in clinical settings. Decide which home pregnancy test you would like based on how sensitive you want the test to be and which format (stick, strip, or digital) you prefer.

At Home Pregnancy Tests listed by Sensitivity

Best Home-Pregnancy Test: First Response Early Result

According to reviews, First Response Early Result is the most reliable pregnancy test, delivering the earliest possible results. The test was able to detect 12.5 mIU of hCG in product testing, making it able to indicate pregnancy on the first day of a missed period 95% of the time in product testing. First Response is available in one-, two-, and three-strip kits.

Best Low-Sensitivity Pregnancy Test: Fact Plus Pro

Women who have hCG in their system want a low-sensitivity pregnancy test. That means that it will only indicate that you are pregnant when you have a very high level of the hormone. Fact Plus Pro detects 100 mIU of hCG and one- and two-strip kits are available.

Best Digital Tests: Clearblue Easy Digital and e.p.t Certainty Digital Home Pregnancy Test

If you are looking to eliminate the guesswork and would like to use a digital test, the Clearblue Easy Digital pregnancy test had been found to be quite reliable. It displays the words "pregnant" or "not pregnant" in a small window. The screen is complicated by the presence of two dye lines (which on standard tests indicates a positive result) on the stick. Apparently this indicates the presence of an unrelated hormone. Clearblue advises women to ignore these lines and focus instead on the digital read out. The instructions are simple: you place the absorbent strip under your urine stream for 5 to 7 seconds, a flashing ‘test ready’ display appears for 20 to 30 seconds to let you know the test is working properly and within 3 minutes the results are spelled out on the window and remain there for up to an hour. Clearblue detects down to 50 mIU of hCG. Clearblue includes two strips so the test can be performed twice; just be sure that you clear the holder of any urine from the previous test before inserting the new test stick. Some negative reviews of Clearblue indicate that some women have gotten a "test malfunction" reading while others received tests that had been assembled incorrectly or the battery was dead. In all cases the device cannot be used.

The e.p.t Certainty Digital Home Pregnancy Test is another, very similar digital test available on the market. As with Clearblue, some of the e.p.t tests are sold with 2 test strips. Some errors do occur; in such an instance a “see leaflet” message appears which indicates that something is wrong and you must wait 30 minutes before trying again.

Now that I’ve taken my home pregnancy test, what should I do now?

You should contact your doctor, nurse practitioner, or midwife if you get positive results on your home pregnancy test or if you have gotten many mixed results. Your doctor can use a more sensitive test along with a pelvic exam to confirm your pregnancy. Seeing your doctor early on in your pregnancy means that you can begin prenatal care.

If your home pregnancy test is still negative two weeks after a missed period, check with your doctor. Many things can cause missed periods, including illness, stress, excessive exercise and hormonal imbalances. Your doctor can help you get your menstrual cycle back on track.

For more information on pregnancy tests, contact the National Women's Health Information Center at 1-800-994-9662 or contact the following organizations:

Food and Drug Administration 
Phone Number(s): (888) 463-6332 
Internet Address: http://www.fda.gov

American College of Obstetricians and Gynecologists 
Phone Number(s): (800) 762-2264 for publication requests only 
Internet Address: http://www.acog.org/

Planned Parenthood Federation of America 
Phone Number(s): (800) 230-7526 
Internet Address: http://www.plannedparenthood.org

American College of Nurse-Midwives 
Phone Number(s): (888) 643-9433 
Internet Address: http://www.midwife.org

Sources:

http://www.4women.gov/faq/pregtest.htm

Pregnancy: Your Guide to Pregnancy Tests

http://www.webmd.com/baby/guide/pregnancy-tests

http://www.consumersearch.com/www/family/pregnancy-tests/comparison.html

http://www.mayoclinic.com/health/home-pregnancy-tests/PR00100

Graph courtesy of:

http://www.early-pregnancy-tests.com/compare.html