Table of Contents
- What Is a Pregnancy Test?
- How Accurate Are Home Pregnancy Tests?
- What Can Cause a False Negative Pregnancy Test?
- What If My Period Is Late and Pregnancy Tests Are Negative?
- When to Call a Doctor
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Today’s at-home pregnancy tests tend to be very accurate when used correctly. However, there are instances—though rare—in which they can produce incorrect results.
False pregnancy tests are usually due to user error, though there are other factors that can skew a test’s results. Read on to learn what can cause a false negative pregnancy test, as well as exactly how these tests work.
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What Is a Pregnancy Test?
A pregnancy test is used to determine whether or not an individual is pregnant. The type of pregnancy test used at home is a urine test, which measures the level of human chorionic gonadotropin (hCG) in a pregnant person’s body. This hormone is only produced when you are pregnant, and it appears soon after the embryo attaches to the uterine wall.
To take an at-home pregnancy test, you either place the test strip in your urine stream, or dip the test strip into a cup of collected urine. When the urine is placed on the test, the antibody in the test recognizes and hooks onto hCG if it is present. The test has a coloring agent that is activated when the hormone is detected, which will result in a line coming up or another indicator of the pregnancy test results, such as a digital screen reading ‘yes’ or ‘no’.
How Accurate Are Home Pregnancy Tests?
The at-home pregnancy tests currently available on the market are “very good tests,”according to Thomas M. Price, M.D., medical director of the Duke Fertility Center and professor of obstetrics and gynecology at Duke University Health System in Morrisville, North Carolina.
“The over-the-counter tests have become very sensitive,” says Dr. Price. “The false negative rate, if done appropriately, is less than 1%.”
If someone waits until the appropriate point in their menstrual cycle and ensures that their urine is concentrated enough at the time of testing, then it’s highly unlikely that an at-home pregnancy test will provide incorrect results, he explains.
However, it’s possible to get a false negative pregnancy test. False positive results are possible as well, though Dr. Price notes that they are “very rare.”
What Can Cause a False Negative Pregnancy Test?
As for what can cause a negative pregnancy test, there are a few potential explanations.
Testing Too Early
False negative pregnancy tests are “almost always” caused by timing, meaning the user is testing too soon, according to Dr. Price.
If someone tests too early in their cycle, the placenta may not have produced enough hCG for the test to detect it yet. This could also occur if someone doesn’t wait long enough after intercourse to take a pregnancy test. It’s recommended you test as soon as you’ve missed your period.
Diluted Urine
Another potential cause of a false pregnancy test is if the urine used for testing is too diluted. If the urine is not concentrated enough, then there may not be enough hCG in it for the test to detect.
Because of this, Dr. Price recommends not drinking any liquids for a couple of hours before taking a pregnancy test. Urine also tends to be less diluted first thing in the morning, as you likely haven’t consumed any liquids overnight.
Too Much or Fragmented hCG Hormone
False pregnancy test results can also occur if there is too much of the hCG hormone, or if the test captures a degraded form of the hormone, called hCG core fragment, which tends to increase as one’s pregnancy progresses.
In the first scenario, when there is too much hCG, the elevated levels of the hormone can essentially overwhelm the test, causing it to produce a false negative. This is referred to as the “hook effect.” The hook effect tends to happen further along in pregnancy, when levels of hCG increase.
The second scenario occurs when there is more of the fragmented hormone present, which, again, is generally when the pregnancy has progressed. In this case, the test may capture the fragmented hormone rather than the intact hormone. However, the test’s signal antibody does not respond to the fragmented hormone, and thus the test’s coloring agent is not activated, leading to a false negative.
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What If My Period Is Late and Pregnancy Tests Are Negative?
If your period is late yet your pregnancy test keeps coming back negative, you’re not alone—Dr. Price notes that this happens very frequently, as there are a number of reasons for late periods that have nothing to do with a potential pregnancy.
“Some women do not always ovulate on a perfectly regular basis,” he says. “And if you don’t ovulate, then you’re going to not have a period or will be late on your period.”
However, if irregular periods are a routine occurrence, then this could signal an underlying issue. “If it’s a chronic occurrence, women that just have irregular periods, there are several medical disorders that can cause that,” says Dr. Price.
This could include disorders such as thyroid conditions and polycystic ovary syndrome (PCOS). Aside from medical disorders, factors like stress, excessive exercise and a low body weight can also cause someone to miss their period.
When to Call a Doctor
If a person is concerned, or has symptoms of pregnancy but is getting negative or irregular test results, Dr. Price says to call the doctor. This is also the case if someone is having difficulty interpreting their test results and can’t tell for sure if it’s positive or negative.
“It’s very easy to call and go in to get a blood test,” says Dr. Price, noting that blood tests are “as accurate as you can get.”
Additionally, Dr. Price advises that anyone who is experiencing chronic irregular periods should see a gynecologist.
Accuracy Matters
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