Pregnancy tests Q&A

Have you ever needed to take a pregnancy test, but you have not been sure how or when best to take one? Here we cover the basic science around pregnancy tests and dispel some myths surrounding them. Read on to find out more.

Q: How do pregnancy tests work?

There are two types of pregnancy tests – urine and blood tests. A urine pregnancy test can be purchased over the counter by anyone of any age. Tests can vary in price, and come as individual testing strips or tests packaged inside a hard plastic case. Urine pregnancy tests are available in both red and blue dye versions, as well as digital versions. 

Figure 1 – Illustration of pregnancy test results by Sofia Miettinen

To take a urine pregnancy test, you can either point the absorbent tip of the test stick mid-stream, or you may find it easier to catch a sample of urine into a clean container. The pregnancy test is dipped into urine for around five to seven seconds. The test is then left to develop for a few minutes, allowing the urine to absorb along with the test and produce both a control (C) line as well as a second test (T) line if the test is positive. If your test only has the control line, the test is negative. If your test has two lines, even if the test line is lighter or darker than the control line, the test is positive. A digital test will show the result in words. If your test has no control line, the test is invalid/faulty and you should repeat it. Different brands of pregnancy tests have slightly different instructions, so you should follow the instructions given with each test.

Blood tests can also check for pregnancy, and can be performed at your GP or booked with a private provider.

Q: What is the science behind a pregnancy test?

Once an egg has fused (been fertilised) by sperm, this produces a group of cells called an embryo. Cells on the outside of the embryo are called trophoblast cells produce a hormone called human beta chorionic gonadotropin (beta Hcg).

Pregnancy tests use monoclonal antibodies which are attached to the pregnancy test, and if they bind with beta hCG, it will cause a reaction to produce a colour change to indicate a positive result.

Q: When should I do a pregnancy test?

Pregnancy tests are typically recommended to be taken when your period is late, or after 21 days of unprotected intercourse if you are unsure of when your next period is due. Some at-home urine pregnancy tests have been shown to be able to detect very small amounts of hCG, meaning you may be able to get a positive pregnancy test as early as 8 days post ovulation (1), which can be up to a week before your period is due. Each brand of pregnancy test can detect different levels of beta hCG so you should check the instructions on your test of when best to test.

Q: Can anything affect the result of my pregnancy test?

A positive pregnancy test indicates that there is beta hCG present in your body. If a test shows a positive result, but you are not pregnant, this is called a false positive. If a test shows a negative result, but you are pregnant, this is called a false negative. False positives can occur if the test is faulty but this is extremely rare. False negatives are much more common, and may occur for various reasons discussed below:

      • Urine concentration

You should take a pregnancy test with concentrated urine, which you may find easiest to do by taking a urine pregnancy test with your first urine sample after waking up from a longer period of sleep. Drinking a lot of water before taking a test can dilute the hCG hormone levels in your urine sample and also cause a false negative.

      • Ovulation and implantation

Despite tracking your period, it is very difficult to identify exactly which day you ovulated. Once ovulation has occurred, it takes on average 8-10 days for the egg to implant into your uterus and begins to produce beta hCG. Implantation can vary for each individual, as well as varying each cycle for the same person. Pregnancy tests measure how much beta hCG is in your body. Beta hCG levels roughly double every 48 hours (2), so it can take time for levels to rise enough to be detected. You can experience a false negative because you have taken the test too early and should take a new test a few days later.

      • Chemical pregnancy

Sometimes you may have a positive pregnancy test a few days before your period is due, but then go on to have your period when you expected. This does not mean you had a false positive, but unfortunately, the pregnancy has miscarried early, sometimes also known as a chemical pregnancy. Because of this, it is recommended to wait until your period is late to take a pregnancy test.

      • Contraception, antibiotics, and alcohol do not affect the result of a pregnancy test.

You may also have heard that a man can use a pregnancy test to check if he has cancer; this is true. As pregnancy tests work by detecting beta hCG, anyone who has beta hCG in their body can have a positive pregnancy test.This does not mean a man is pregnant, but that something is causing beta hCG to be produced, which some types of testicular cancer can cause. 

Q: What should I do if I have a positive pregnancy test?

There are several reasons why you may have taken a pregnancy test, and you can contact your GP or community sexual health clinics for further advice on the next steps available to you. More information is provided on the NHS website: Doing a pregnancy test – NHS.


  1. Cole LA, Sutton-Riley JM, Khanlian SA, Borkovskaya M, Rayburn BB, Rayburn WF. Sensitivity of Over-the-Counter Pregnancy Tests: Comparison of Utility and Marketing Messages. Journal of the American Pharmacists Association. 2005 Sep;45(5):608–15.
  2. Fritz MA, Guo S. Doubling time of human chorionic gonadotropin (hCG) in early normal pregnancy: relationship to hCG concentration and gestational age. Fertility and Sterility. 1987 Apr;47(4):584–9.

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