Pregnancy tests can return false positives or false negatives. What are the reasons? What must be considered? Here we detail it.
Have you heard about false negatives and false positives in a pregnancy test? Do you know what each one refers to? Well then, the starting point is that failures in these tests They’re possible. Although they currently have high sensitivity and specificity, sometimes they do not give an accurate result.
To understand it better, remember that these tests detect a hormone known as ‘human chorionic gonadotropin (hCG)’. It is secreted through the placenta from the beginning of the pregnancy; Furthermore, it is possible to detect it both in urine and in blood. Its levels increase as pregnancy progresses.
However, there are some situations where the test is faulty or have errors in their results. Thus, it is possible to obtain a positive result without actually being pregnant (false positive), or a negative one despite being pregnant (false negative). We tell you more about it.
What is human chorionic gonadotropin?
Failures in the pregnancy test occur due to erroneous readings in the levels of the human chorionic gonadotropin hormone. Often the ‘beta’ subunit of this hormone is measured, which is why the pregnancy test is called a ‘beta-hCG’, for its acronym in English.
Normally, the levels of this hormone increase progressively as the pregnancy progresses, especially in the first trimester. These double every 48 to 72 hours, and reach a maximum peak around the 12th week of gestation.
hCG itself is a chemical produced largely by the syncytiotrophoblastic cells of the placenta during pregnancy.. Its function is to stimulate the corpus luteum to produce the progesterone necessary to maintain the pregnancy.
It can be detected in both blood and urine. However, blood tests are considered more sensitive because they detect lower levels of the hormone and allow for qualitative and quantitative measurement.
- Qualitative measurement identifies whether there is presence or absence.
- Quantitative measurement detects levels.
Meanwhile, urine pregnancy tests are homemade and require higher levels of the hormone for detection. Added to this, they depend on the mode of use given by the user. Therefore, if there are errors in its implementation, there is also a greater chance that it will fail.
It is important to mention that the hormone is detected between 14 and 17 days after conception. Therefore, it is recommended to wait 15 days after having unprotected intercourse or at least one day of menstruation to obtain a more accurate result.
False negatives in the pregnancy test, one of the frequent failures
False negatives are the most common pregnancy test failures. They can occur due to different situations, such as the following:
- Very early stages of gestation. It is recommended to wait one to three days after the delay in menstruation or at least 15 days after intercourse without contraceptive protection.
- very dilute urine. This can lead to very low human chorionic gonadotropin hormone levels. Therefore, it is recommended to use the first urine in the morning.
- Incorrect use of the pregnancy test. Such as the error in the place of urine placement or exposure time. It is essential to follow the instructions to the letter.
Now, if the result is negative, but there are still doubts or the menstruation does not appear, the test should be repeated after five or six days (or at least two days apart) to corroborate the result.
Failing that, you can also take a blood pregnancy test or see your doctor (who may suggest a transvaginal ultrasound).
false positives in pregnancy test
When the pregnancy test comes out positive, there are over 97% chances that it is true. Even so, sometimes there are failures and false positives can result. This usually occurs when there are situations where the human chorionic gonadotropin hormone is elevated without pregnancy. For example:
- Some germ cell cancers (gonadotropin-secreting tumors).
- Trophoblastic disease (choriocarcinoma).
- Hormonal treatment for assisted reproduction processes (Human chorionic gonadotropin hormone is sometimes used as part of the therapy.)
- Consumption of some medicines, such as some anticonvulsants, diuretics or anxiolytics. For this reason, the medication leaflet should be read and the doctor informed about its use.
Likewise, it can occur in other cases of non-viable pregnancies such as the following:
- Ectopic pregnancy, which occurs outside the uterus and the chances of favorable development are low.
- Anembryonic or anembryonic (without an embryo) pregnancy.
- After a miscarriage.
After an abortion, there is a progressive decrease in the hormone human chorionic gonadotropin. Therefore, it can take weeks until it is undetectable. Hence, the test yields positive results in the absence of pregnancy.
However, it is necessary to follow up, because if the levels do not drop after several days, may indicate that embryonic remains remain in the uterine cavity. If so, the doctor must intervene.
Ultimately, when the home pregnancy test is read outside the recommended time, it can return false positives. The same happens if the test is expired.
Pregnancy test failures can be avoided with correct use
The most important thing to avoid pregnancy test failures is to use it properly and use the most sensitive one possible. On the other hand, It must not be done before the indicated date. This applies especially to women undergoing assisted reproduction treatments, since they can yield false positives.
That said, it’s essential to test when your doctor tells you to. In addition, it is convenient to follow all the instructions step by step. Most of the time, failures occur due to the use of expired tests or due to poor performance.
In summary, some recommendations are as follows:
- Repeat the test three to five days later.
- Do not confirm the test after the time indicated in its instructions has elapsed.
- Consultation with a gynecologist after 4 to 5 weeks.
- Prefer the blood pregnancy test with quantitative quantification.
Whenever a false negative is suspected, a blood pregnancy test will be requested. In addition, a transvaginal ultrasound will be performed in search of the gestational sac and the embryo.
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All cited sources were thoroughly reviewed by our team to ensure their quality, reliability, currency, and validity. The bibliography for this article was deemed reliable and of scholarly or scientific accuracy.
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