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Jhoana Gabriela

Pregnancy test: everything you need to know about false positives

Updated: Oct 8

The pregnancy test determines whether a person is pregnant or not. Its effectiveness is based on detecting the hormone human chorionic gonadotropin, or HCG, which begins to be produced upon implantation of the fertilized egg.


There are many reasons why a person may want to check if they are pregnant. Whether you want to have a baby, have stopped your birth control, have irregular periods and have doubts about a possible pregnancy, or have had an abortion and want to confirm if it was completed, it's important to know how the different types of each pregnancy test available work.


In this guide, we have compiled all the facts you need to get a reliable result. Yes, whether you are a cisgender woman, a transgender man, or a non-binary person, you will find information here to answer your questions!


What is the home pregnancy test


If your menstruation is late and you also have other symptoms such as fatigue, nausea or breast tenderness, you can use a home test, which makes an early detection of pregnancy through urine.



These tests are convenient, inexpensive, and easily available in pharmacies, without the need for a prescription. Typically, the concentration of HCG in the blood and urine increases exponentially during the first trimester, doubling every 24 hours for the first 8 weeks.


What you should know when taking a home pregnancy test:


  • It is recommended to wait until after the day your period is due.

  • Read the instructions carefully and follow them step by step.

  • Perform the test in the morning. That is the time when your HCG levels are more concentrated, so the result is more reliable.

  • Do not drink too much water before taking the pregnancy test, as the result may be altered.

  • Make sure that the expiration date of the test has not passed.


Are the results of a home test reliable?


When properly performed, these tests work 99 out of 100 times. However, some factors may affect the accuracy of the result.


If the result is positive, it indicates the presence of a fertilized egg, and the individual will need to consider next steps, whether or not she decides to continue with the pregnancy.


False positives are rare, but may occur in cases where blood or protein is present in the urine. In addition, certain medications, such as aspirin, carbamazepine, and methadone, can generate a false positive as well.


If the result is negative, it indicates that there is no pregnancy.


Things you should NOT do because they increase the risk of false positives


  • Test at night. Morning urine has a higher concentration of HCG than evening urine, when urine is usually more dilute.

  • Using an expired test. Check the expiration date on the packaging.

  • Drinking plenty of water before testing. Drinking too much fluid can dilute the urine, making it difficult for the test to detect the presence of HCG if you are in the early stages of pregnancy.

  • Not following the instructions correctly. Each test may have its own variations in use and reading of results.

  • Doing the test before menstruation is late. 

  • Not checking the results within the stipulated time. Most tests are specific in that the valid result is read within 5 to 10 minutes. Outside that time it may give an erroneous result.

  • The Hook Effect (Hook Effect) can also give a false negative, because the amount of HCG produced by the body is so high that it saturates the antibodies in the test, and they cannot function properly to give a correct result.


Tip: Repeat the home pregnancy test one week after the first attempt. By waiting a few more days, you increase your chances of getting a more accurate result, as your body will have produced higher levels of the HCG hormone if pregnancy is present.


Blood tests to detect pregnancy


Since the effectiveness of home pregnancy tests can be lowered by up to 75% due to errors in handling, blood tests are an option with an effectiveness of 99%.


Blood tests are performed in a medical laboratory, so they tend to be more expensive than home pregnancy tests. This type of test uses a small blood sample to detect the presence and amount of HCG. Currently, there are two types:


  • Qualitative HCG blood test, which only determines whether it is positive or negative for pregnancy.

  • Quantitative HCG blood test, which not only identifies the presence of the hormone, but also measures its exact level. This measurement is key to assessing the progress of the pregnancy and detecting possible complications. For example, the physician may use this test, along with other tests, to rule out ectopic pregnancy, a condition in which the fertilized egg implants outside the uterus.


Is it necessary to perform an ultrasound?


To detect pregnancy, a blood or urine test is sufficient. Ultrasounds, also known as ultrasounds, are studies that use sound waves to obtain relevant information about the pregnancy, such as the weeks of gestation, the growth and development of the fetus, and the state of the pelvic organs. Thus, these studies are not recommended as a method of pregnancy detection, but as an integral part of prenatal care. 



In the case of Voluntary Interruption of Pregnancy (VTP), the ultrasound scan determines the weeks of gestation, which is essential for choosing the appropriate method of termination. Finally, ultrasounds are essential to identify ectopic pregnancies. Misoprostol, a medication that induces abortion, is not effective when a person has an ectopic pregnancy. In this case, medical attention and treatment by a physician is necessary. Read more here.


Your body, your decision


When the pregnancy is confirmed, it is essential to evaluate your options. If you decide to continue with the pregnancy, it is advisable to schedule an appointment with your specialist to start prenatal care.


On the other hand, if it is an unwanted pregnancy, you have the option to consider the Voluntary Interruption of Pregnancy (VTP), in the Aya Contigoapp , you can find valuable information, sorora and evidence-based to guide you in this process, as well as a team to clarify your doubts and accompany you.


Visit our forum to post your questions and experiences about abortion and contraceptives.

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