Science and curiosity: How does the pregnancy test work?

by Antonino Napoleone, Maria Chiara Rosace | October 28, 2021.

The pregnancy test is a widely used tool for reliable confirmation of whether or not a conception has occurred. Previously, ScienceRely has covered topics related to the female reproductive system and fertilisation (read more: EllaOne and emergency contraception and What to know about the morning-after-pill?), but this time the focus is on a diagnostic tool of considerable medical and social importance. In fact, it is a test whose results can have a huge impact on a couple’s life, health and status. Why is it called a rapid test and how is it so accurate and reliable? And above all, why is it performed on a urine sample? Driven by curiosity, let’s try to answer these simple questions and understand how the pregnancy test works!

The pregnancy hormone

There are different types of pregnancy tests, but in general, they are all based on the detection of a specific hormone called human chorionic gonadotropin (HCG). This hormone is detectable in the woman’s urine or blood from the first few weeks after conception. HCG is the main hormone produced by the inner layer of the placenta called the chorionic layer or ‘chorion‘. The placenta is the temporary organ of the female reproductive system that is generated during pregnancy to provide protection and nourishment for the developing foetus. In addition, the placenta performs an important function in secreting and regulating hormone levels during pregnancy. Indeed, from 6 to 12 days after conception, HCG hormone levels increase significantly after implantation of the fertilised egg in the woman’s uterine wall.

When is the pregnancy test positive?

The main characteristic of the HCG hormone is its structure consisting of two parts or subunits. The first is the alpha subunit (α-HCG), which is similar in structure to other female hormones belonging to the class of gonadotropins, such as LH (luteinizing hormone) and FSH (follicle-stimulating hormone). While the second is the beta subunit (β-HCG) which, unlike the first, is highly specific and shares no similarities with other hormones.
Domestic rapid pregnancy tests are commonly available (Figure 1). They are very accurate and sensitive in determining whether or not a pregnancy is in progress because they are able to detect the presence of the β-HCG hormone in the woman’s urine. When used correctly, domestic pregnancy tests have an accuracy rate of over 98% 7 days after implantation of the fertilised egg.

How does the test work?

Rapid pregnancy tests can be purchased in pharmacies. They consist of a characteristic plastic stick with a reactive strip in the centre containing specific molecules produced in the laboratory, which are designed to recognise only the β-HCG hormone. These molecules are monoclonal antibodies specific for the β-HCG hormone, which through a chemical reaction, in about 10-15 minutes after use, provide a positive or negative result that can be read by means of coloured stripes in the central part (figure 2).

The test is carried out by placing the absorbent tip in the urine stream for about 5 seconds. Alternatively, urine can be collected in a dedicated sterile container, where the absorbent tip can be placed. It is essential to check the expiry date and instructions on the package. A positive result from the domestic test requires further confirmation with further tests carried out in specialised laboratories, where it is possible not only to check for the presence or absence of the HCG hormone (qualitative result) but also the levels in the blood and urine (quantitative result) with greater accuracy, in order to obtain all the necessary information about the early stages of pregnancy.

References

1. Peter Berger, Catharine Sturgeon, Pregnancy testing with hCG – future prospects, Trends in Endocrinology & Metabolism, Volume 25, Issue 12, 2014, Pages 637-648, ISSN 1043-2760, https://doi.org/10.1016/j.tem.2014.08.004.
2. Chard T. Pregnancy tests: a review. Hum Reprod. 1992;7(5):701-710. doi: https://doi.org/10.1093/oxfordjournals.humrep.a137722.  
3. https://medlineplus.gov/lab-tests/pregnancy-test/
4. https://www.britannica.com/science/pregnancy-test