How Does a Pregnancy Test Work?

Since the earliest record of urine-based pregnancy tests in Ancient Egypt [1], there have been plenty of breakthroughs, most importantly, the introduction of at-home based tests in 1976 - more information about the history of pregnancy tests here.

All the reliable modern pregnancy tests are based on the human chorionic gonadotrophin hormone (hCG), which appears at a very early stage of pregnancy. This is because the placenta, which develops shortly after a fertilized egg attaches to the endometrium, produces hCG [2].

The concentrations of hCG start increasing 7–10 days after the LH peak, or 4–7 days after implantation [3,4] (check this post about the menstrual cycle for more information). During early pregnancy, hCG concentrations increase exponentially as the placenta develops, doubling on average every 1.5–2 days, and reaching its peak at 7–10 weeks of pregnancy. The concentrations level out at 13–15 weeks, increase moderately until weeks 30–33, and then decrease towards the end of term [3].

Urine test

Urine-based pregnancy test works similarly to COVID-19 rapid tests (more information here). There are four parts in the pregnancy test strip:

1. Pad where urine is deposited

2. Antibodies* specific to hCG

3. Test line containing another kind of antibody also specific to hCG that are attached to the strip and unable to move.

4. Control line that contains non-hCG antibodies also attached to the strip and unable to move.

After urine is collected, hCG (if there are any) will bind to the antibodies and move together towards the test line. If there is hCG, the aforementioned group will attach with the antibodies from the test line and this will trigger a colour change, forming the line. The control line contains non-hCG antibodies that will attach to the free antibodies from earlier. This line is important to ensure that the test is working properly. Therefore, there will always at least be one line present, whether there is a pregnancy or not. If there is no line, that means the test was defective and another should be performed.

Blood test

The blood test detects the quantity of hCG [2] with a technique called electrochemiluminescent immunoassay [6] (ECLIA). This technique works very similarly to the previous one, where the hCG will attach to an antibody, but in this case the union between antibody and hormone will generate a light. This light can then be measured with a specific machine in the laboratory and used to calculate the amount of hCG in the blood [7].

Comparison

Even though both tests check hCG levels, the urine test only indicates whether one is pregnant or not while a blood test shows how much hCG is present in the body.

There are advantages and limitations to both urine and blood tests. Blood tests are more consistent and are not as affected by liquid intake as urine results can be. However, false positives can happen due to non-hCG antibodies interfering with the test. False negatives can happen with urine tests, occurring when the antibodies attach to non-hCG proteins found in urine, which can happen at any point during pregnancy [8]. Furthermore, urine tests can be taken at home, while blood tests are also more expensive and time-consuming due to getting blood drawn at a facility [2].

Stay turned to learn more about women’s health - specifically, how stress can cause menstrual irregularities!

*Antibodies - a molecule that recognized and binds to another molecule (antigen).

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Written by: Phaedra

Edited by: María and Natasha

BioDecoded is a volunteer group committed to sharing accurate scientific information. If you have any questions about this topic or would like to learn more, please comment below or send us your questions.

References:

1. Leavitt S. (2006). A private little revolution: the home pregnancy test in American culture. Bull Hist Med 80:317–45. Available at: https://pubmed.ncbi.nlm.nih.gov/16809866/

2. Pregnancy Test: When To Take, Types & Accuracy (2023). Cleveland Clinic. Available at: https://my.clevelandclinic.org/health/articles/9703-pregnancy-tests (Accessed: 10 February 2023).

3. Ferreira HP. (1954). The relative merits of the various biological tests for pregnancy. Postgrad Med Journal P355-359. Available at: https://pubmed.ncbi.nlm.nih.gov/13177232/

4. Gnoth C. Johnson S. (2014). Strips of Hope: The accuracy of home pregnancy tests and new developments. Geburtshlife und Frauenheilkunde 74(7): 661-669. Available at: https://pubmed.ncbi.nlm.nih.gov/25100881/

5. How do pregnancy tests work? (2018). Compound Chem. Available at: https://www.compoundchem.com/2018/11/09/pregnancy-tests/ (Accessed: 10 February 2023).

6. HCG - Serum, Quantitative (2023). Iowa University. Available at: https://www.healthcare.uiowa.edu/path_handbook/rhandbook/test446.html (Accessed: 10 February 2023).

7. Zhu, X. and Gao, T. (2019) "Spectrometry", Nano-Inspired Biosensors for Protein Assay with Clinical Applications. Elsevier, pp. 237-264. Available at: https://www.sciencedirect.com/science/article/pii/B9780128150535000106

8. Stenman UH et al (2006). The classification, functions and clinical use of different isoforms of HCG. Human reproduction update 12 (6); pp. 769-784. Available at: https://pubmed.ncbi.nlm.nih.gov/16877746/