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What Your Home Pregnancy Test Can and Can’t Do

Monday, April 28th, 2014

If you have just noticed that your period is late, your first inclination may be to run out and buy a home pregnancy test (HPT). With various brands now purporting to be effective as early as the first day of your missed period, it’s understandably tempting. However, be aware that these claims may not always be exactly accurate. You can improve your odds of getting an accurate reading by being familiar with when and how to take one of these tests, but understand that HPTs are no stand-in for the reliability of a test administered by a doctor.  These tests are from the urine and are imprecise.  The pregnancy tests taken from a blood sample are more accurate and will allow your gynecologist to see a “trend” in the amount of hormones in your system, in case there is an equivocal result.

How Soon Can You Take a Home Pregnancy Test?

Don’t get too excited about those tests that claim to work before your period is even late. Wait until a little later for the best results – once your period is a week late, go ahead and test. By this time, if you are pregnant, you’ll have enough HCG (human chorionic gonadotropin, the hormone produced in your body when you become pregnant) in your blood for the test to detect. Earlier than this, some HPTs aren’t precise enough to detect the smaller amounts of HCG present in the first days of pregnancy.

False Positive/False Negative Results

There are several factors that could interfere with the results of an HPT, including the design of the test itself, taking the test too early, and certain medications such as fertility drugs. Both false negatives and false positives are possible, but false positives are much rarer.

A false positive could occur after taking a medication such as a fertility drug that contains HCG, or it could mean that there was a pregnancy that was lost very soon after the fertilized egg had attached to the uterine lining. An ectopic pregnancy may also produce a positive HPT result, and this requires immediate medical attention. Most often, however, a positive result indicates a normal pregnancy. Either way, a positive result warrants an appointment with your gynecologist.

A false negative result is more likely than a false positive. This means that your HPT will indicate that you aren’t pregnant when you actually are. This can be life-threatening, especially if you are pregnant in one of your fallopian tubes or your cervix or your abdomen (ectopic pregnancy).  The pregnancy may rupture in the fallopian tube or cervix and cause hemorrhage leading to death. You may end up with a false negative if you take the test too early, check the results too soon (without following the package directions explicitly), or using urine that is diluted – for example, if you have recently drunk a lot of water. That’s why even though many HPTs claim to be accurate at any hour, your best bet is to take the test first thing in the morning, when more concentrated urine boosts your chances of getting an accurate result.  In the final analysis, you need to have the result confirmed in a doctor’s office.

What to Do After Taking the Test

If the test is positive, make an appointment with your gynecologist to confirm the pregnancy with a pelvic exam and a blood test.  Of utmost importance is to confirm that the pregnancy is actually in your uterus and not an ectopic pregnancy where it has an increased risk of rupture and hemorrhage. The sooner prenatal care begins, the better, so make this call immediately.

If your test is negative, and you have missed a period, you need to make an appointment to see your gynecologist as soon as possible. There are lots of reasons why you may miss periods, including stress, weight loss, strenuous exercise, and illness. However, if you missed a period, you are pregnant until proven otherwise.    Read more about pregnancy tests and early pregnancy in my women’s health book, “Inside Information for Women”.


– Yvonne S. Thornton, M. D., M. P. H