Interpreting Faint Lines on a Pregnancy Test

woman taking pregnancy test
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Many women repeat home pregnancy tests more than once, and then grow concerned if the result changes from positive to weakly positive. Is it a miscarriage, an error, or something else?

If you had a dark line before and now you have a faint line there could be several explanations for this, other than miscarriage. You may have consumed too much water prior to the test, or not enough. You may have accidentally used the test incorrectly, or something else in your urine may have interfered with the test.

How a Home Test Works

Home pregnancy tests are based on the detection of the pregnancy hormone human chorionic gonadotrophin (hCG) in your urine. When hCG is detected, the test will return a positive result. When hCG is not detected, the test comes back negative. A positive result is usually indicated by two lines, either side-by-side or in a plus sign.

Depending on whether you take the test as instructed, the results may sometimes be uncertain or incorrect. A review study found that in 1982, pregnancy tests had a nearly 25% rate of false negatives, and only one-third of users reported reading test instructions. In the early 1990s, technology increased testing reliability to 99% sensitivity, with 76% of users reporting that they read test instructions.

User Error

While today's tests are more foolproof, user mistakes can still occur during urine collection for the test, as well as when interpreting results. Some tests are performed by:

  • Placing a test stick into the urine mid-stream while urinating
  • Urinating into a cup and dipping a test strip into the urine
  • Urinating into a cup and using an eyedropper to place a small amount of urine onto a testing device

In one study, a hundred women were asked to test all three methods. The third test, using the eyedropper, failed to provide a result 24% of the time. And more than 30% interpreted the eyedropper and strip tests differently than trained study coordinators.

While the stick test only failed to provide a result 5% of the time, and the women and researchers agreed with results 99% of the time, user error is still possible if a user does not fully saturate the strip on the stick. It is generally designed to collect urine under a stream for 7 to 10 seconds. Small variations can make a difference as to whether a test is accurate or inaccurate.

What a Faint Line Means

Although hCG levels increase exponentially during early pregnancy, that doesn't mean that the pregnancy test line will necessarily get darker as each day passes. Things are not always that simple.

Although the line should be darker in relation to higher concentrations of hCG, the actual amount of hCG in a sample of urine can vary. This is due in large part to the concentration of your urine at any given time. This can fluctuate dramatically throughout the day depending on how much fluid you drink and how frequently you go to the bathroom, among other things.

The more you drink, the more diluted the hCG in your urine will become. The same can occur if you urinate frequently and constantly excrete hCG from your body.

As a general rule of thumb, if your urine is light or clear, it is less concentrated and may not produce a distinctive result on a home pregnancy test. In theory, if you have darker urine, you may get a more distinguishable line than if you test with lighter urine a day or two later.

Because of this, taking multiple tests to check the color of the line is not a reliable means of monitoring the development of early pregnancy. Even though you will likely pass more hCG in your urine as the pregnancy progresses, a home test will not necessarily reveal a darker line hour-by-hour or day-by-day.

Urine vs. Blood Tests

Blood is far more reliable for monitoring hCG levels. That's because blood maintains a fairly steady composition at all times, thus making it easier to monitor changes in chemicals such as hCG. That's why quantitative hCG blood tests over a period of days are far more reliable that urine tests for monitoring hCG levels.

Blood tests can detect pregnancy nine to 10 days after ovulation. However, your doctor won't suggest a blood test until after your period is due and only if there is a medical need.

If you have had pregnancy complication in the past or are worried about miscarriage, don't waste money buying multiple home pregnancy tests. Instead, speak to your doctor about a blood test to confirm the pregnancy and ensure that it is progressing as it should.

Changes in Test Results

If you get a negative result on a home pregnancy test after having gotten a positive result, this could be an understandable cause for concern and confusion. While it is may simply be a false negative (usually as a result of a user error), it could also be the result of a very early miscarriage, known as a chemical pregnancy.

A chemical pregnancy is one that occurs before the fifth week of gestation and well before the fetus can be visibly detected on an ultrasound. It is believed to affect as many as 25% of pregnancies that end in miscarriage.

If a chemical pregnancy is suspected, your doctor may test the hCG levels in your blood several times. Decreasing levels may provide evidence that a chemical pregnancy has, in fact, taken place.

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