top of page
  • Writer's pictureC.A.R.E. Medical Staff

How a Pregnancy Test Works

** All of C.A.R.E. Medical Center's healthcare related content is reviewed by a healthcare professional prior to publishing **

Have you ever taken a pregnancy test and wondered how exactly this little stick tells you that you are pregnant? Maybe you are wondering if you are pregnant and are thinking about taking a test. Maybe you took a home test but aren’t sure if you can trust the answer. Advancements in medical technology have led to the development of highly accurate medical-grade pregnancy tests which you can use both at home or have be conducted by a medical professional. By exploring the science behind medical-grade pregnancy tests, we can understand how exactly this whole business works and through greater knowledge have greater confidence in making your healthcare decisions.

How a Pregnancy Test Works

Medical-grade pregnancy tests work by detecting the presence of human chorionic gonadotropin (hCG), a hormone produced by the placenta shortly after the implantation of a fertilized egg. The concentration of hCG increases rapidly during early pregnancy [1]. There are two primary ways of retrieving a sample which is then tested for hCG, through a urine test (which is the method home pregnancy tests use, but also medical grade tests) as well as blood tests.

Levels of hCG in the body double every two to three days, which is why waiting later to take a test, if you suspect you may be pregnant, will deliver more reliable results. [2]

Urine-Based Pregnancy Tests Urine-based pregnancy tests are designed to have a small strip of material that collects a sample which then will tell you if you are pregnant or not, based on how the material responds to the levels of hCG in your urine sample. Here’s how it works. The test strip or cassette which makes contact with the urine sample contains antibodies that specifically bind to hCG molecules present in urine. These antibodies are often conjugated with a colored dye or indicator [3].

When urine is applied to the test, it travels through the test strip or cassette. If hCG is present in the urine, it binds to the antibodies on the test line. The test line includes a substance that reacts with the hCG-antibody complex and if there is a certain amount of hCG in the urine sample this will cause a reaction in this substance which results in a visual change such as the appearance of a colored line or symbol, indicating a positive result [3].

Now in order to verify the test's accuracy, a control line is also incorporated on the test strip or cassette. This control line should always appear as a reference point to confirm the test was conducted correctly.[3].

Blood-Based Pregnancy Tests

Blood-based medical-grade pregnancy tests, typically performed in clinical settings, offer highly accurate results and are more sensitive than urine tests. Urine-based tests are what we call qualitative, this means that they provide a yes or no answer to the question of if you are pregnant or not. Blood-based tests are uniquely quantitative which means that when you process a blood pregnancy test, the results can tell you all sorts of information about not just your pregnancy, but also even your baby! Here’s how they work.

Blood tests measure the concentration of hCG in the bloodstream, which provides a quantitative result that can determine the stage of pregnancy and monitor hCG levels over time. How is this done? Well, a blood sample is collected and sent to a laboratory for analysis. Specialized equipment, is used to precisely measure the concentration of hCG in the blood [4].

The laboratory then provides a detailed report indicating the measured hCG levels which healthcare professionals can interpret the results from and offer further guidance based on the individual's specific circumstances [4].

So, What's the Difference Between the Two?

Like we said the answer to a urine test is yes or no, but the answer to a blood test is much more detailed. Is it better to have your pregnancy confirmed by a blood test instead of a urine test then? Not necessarily. Firstly, urine tests are incredibly reliable, within 98%-99% accurate, secondly, urine tests are significantly faster than a blood test, generally only taking a handful of minutes to render a result, as opposed to days for a blood test, furthermore the information in a blood test may only be relevant in certain cases (e.g. high-risk pregnancy, hereditary conditions, etc.).

We conduct urine tests here at C.A.R.E. Medical Center primarily because that’s just what makes sense for our patients. Now we are equipped to offer blood-based tests to our patients if they have missed a period and are demonstrating signs of pregnancy but have received multiple negative pregnancy tests. If you think you should have a blood test though we would encourage you to talk about that with your general practitioner or our patient advocate, who can help you work through that question.

Why Should I Get a Medical Grade Pregnancy Test?

Home tests are practically as accurate as medical grade tests [2] – when they are conducted correctly. A number of factors, typically coming from human error can result in a faulty test. A likely typical error is taking a test too soon and getting a false negative. Remember that levels of hCG rapidly increase as pregnancy continues so it’s best practice when taking an at-home pregnancy test to wait until after the first missed period. Even then some pregnancy tests are sensitive to the time of day when the sample is collected. It’s recommended to collect a urine sample first thing in the morning.

Since medical-grade pregnancy tests are administered by a healthcare professional in a sterile clinical setting you can have confidence in the clinical tests answer. Healthcare professionals are equipped to correctly and safely handle and process tests, resulting in a reliable interpretation of results. In addition to this, medical grade tests typically are more sensitive to the amount of hCG that they can detect in comparison to at-home tests. As such times when an at-home test may not be able to reliably give an answer, clinical tests can.

Clinics like C.A.R.E. Medical Center exist to give women peace of mind by having a medical professional verify their pregnancy. When you come into our clinic, not only can you be sure that you will receive the best care possible but also that whatever the result, our staff will be there with you to help navigate whatever the future looks like.


If you or someone you know is facing an unplanned pregnancy, we encourage you to reach out to our organization for help. You can set up an appointment today by emailing or calling (360) 428-4777. We are open Monday-Thursday and accept walk-ins. We are located at 110 n 15th St Mount Vernon WA. All of our services are FREE and CONFIDENTIAL.


  1. The Cleaveland Clinic, n.d."Human Chorionic Gonadotropin"

  2. Bass Medical Group, 2020, "Pregnancy Testing at the Doctor's Office: What You Need to Know"

  3. Compound Interest, 2018 "How do Pregnancy Tests Work?"

  4. Grenache, David G., 2011 "Human Chorionic Gonadotropin"

74 views2 comments
bottom of page