Although many home pregnancy tests claim 99 percent accuracy, they are not perfect. A false-positive result is one where the test indicates that the user is pregnant when she is not.
Several factors can lead to a false-positive or a negative reading. This article discusses some of the potential causes of a woman getting a false-positive pregnancy test reading.
Contents of this article:
How do home pregnancy tests work?
Home pregnancy tests rely on urine to detect the presence of hormones released during pregnancy.
Home urine pregnancy tests work by detecting the presence of human chorionic gonadotropin or hCG in the urine. This is the hormone of pregnancy, and it is found in both the urine and blood.
Home pregnancy tests vary depending on the brand, and each one will have detailed instructions to follow for accuracy.
Some kits require the user to place a test stick in her urine stream. Others may use a urine collection cup to either dip a pregnancy stick in or to retrieve urine from with a dropper for placement on the test stick.
The way the test reveals a positive or negative result also depends on the brand and can include:
- a plus or minus sign
- the word or words "pregnant" or "not pregnant"
- one or two indicator lines
Importantly, when taking a pregnancy test, a woman should be aware that even a faint plus sign or a line indicates a positive result.
However, not all positive pregnancy tests mean that a woman is pregnant. There are some occasions when a pregnancy test can give a false-positive reading.
People should speak with their doctor for further evaluation if they receive a positive pregnancy test.
Also, it is important for a woman to speak with a medical professional if she believes she may be pregnant, but she has received a negative pregnancy test. This is because further testing may be necessary to confirm her condition correctly.
Listed below are five possibly situations which could cause a woman to have a false-positive result from a home pregnancy test.
1. Evaporation lines
When taking a home urine pregnancy test, it is important to follow the test's instructions fully. Each brand has specific instructions, so it is vital to read them closely.
Most tests will ask the user to read the result within 4–5 minutes of taking the test and no longer than 10–30 minutes afterward.
Most non-digital urine pregnancy tests will show one line if a woman is not pregnant and two lines if she is pregnant, or a plus or minus sign.
If the pregnancy test is read after the recommended time has elapsed, it may result in what looks like a positive test result. However, in most cases, this is not a positive pregnancy test but an evaporation line left by the urine after the recommended time for reading the test.
2. Previous miscarriage or abortion
After an abortion or miscarriage, a home pregnancy test may produce a false-positive result, since the levels of hCG in the body may still be relatively high.
Taking a home urine pregnancy test, following an abortion or miscarriage, can be another reason for a false-positive result.
Once a fertilized egg is implanted into the uterine wall, the body begins to secrete the pregnancy hormone hCG. After a miscarriage or abortion, the levels of hCG start to fall, but only slowly.
Typically, hCG declines over a period ranging anywhere from 9 to 35 days. The average time frame is around 19 days. Taking a pregnancy test within that period can result in a false-positive test.
At times, a woman may have experienced an incomplete miscarriage. This condition means that there may be tissue from the pregnancy in the uterus that continues producing hCG. If this occurs, surgery with a dilation and curettage may be needed, to remove any retained products of conception.
3. Molar pregnancy
A molar pregnancy or hydatidiform mole is a condition that causes a uterine tumor to grow.
A molar pregnancy is caused by genetic abnormalities at conception and results in an abnormal pregnancy.
In a complete molar pregnancy, the mother's chromosomes are not present, only the father's. This leads to no embryo or placental tissue.
Conversely, a partial molar pregnancy has an embryo and a placenta. However, the embryo has two sets of chromosomes from the father and one set from the mother. This gives the embryo 69 chromosomes instead of the normal 49.
Instead of a normal placenta and embryo, the placental tissue in a molar pregnancy develops in a way that resembles a bunch of grapes. This results from a mass of small fluid-filled sacs forming, which must be removed with a dilation and curettage.
A woman's hCG levels are closely monitored, following surgery, to be sure they return to normal.
At times, a condition called persistent gestational trophoblastic disease can occur. This is caused by the continued growth of the products of a molar conception within the uterus. It can be a serious medical condition requiring surgery to remove the womb or chemotherapy.
Women who have molar pregnancies can still grieve the loss of their pregnancy, similarly to women who have miscarriages.
Certain medications may lead women to have false-positive pregnancy tests.
These medications include those with hCG as an active ingredient, which is often used to treat infertility. A person who is taking medication to treat infertility may wish to be tested for pregnancy by their doctor.
Most medications and drugs, including alcohol, do not affect the results of home pregnancy tests.
5. Medical conditions
Various medical conditions can affect the level of hCG in the body, resulting in a false-positive pregnancy test result.
Certain medical conditions can cause a woman's hCG to rise, even when she is not pregnant. They include:
- disorders affecting the pituitary gland and hormone levels, specifically in perimenopausal or menopausal women
- gestational trophoblastic diseases, which cause tumors in the cells that would normally make up the placenta
- cancers of the ovary, bladder, kidney, liver, lung, colon, breast, and stomach
- phantom hCG, where antibodies interfere with the testing kit
- ovarian cysts
- kidney disease or urinary tract infections
Phantom hCG occurs when certain antibodies bind with the molecules in the pregnancy testing kit that are used to detect hCG. The interaction causes the testing kit to signal that hCG levels are higher than they actually are.
How accurate are home pregnancy tests?
According to United States's Office on Women's Health, home pregnancy tests can be 99 percent accurate when used correctly.
The amount of hCG present in a woman's urine increases with time, and an accurate result is usually obtained if the test is taken after a missed period.
However, in around 10 percent of women, hCG levels only begin to rise the first day after a missed period. In these women, a test is unlikely to be accurate if carried out at this time.
Not using the test the proper way or checking the results later than the recommended time can lead to false-positive results.
What to do if a pregnancy test is positive
If someone has a positive pregnancy test, it is important to notify their doctor, as they will need further evaluation and testing to confirm and monitor their pregnancy.
If someone believes that they may be pregnant, but their home pregnancy test results are negative, they should speak with a doctor for further guidance.