Pregnancy Tests - How Do They Work and When Should I Have One?

At a Glance:

  • Modern pregnancy tests work by detecting the presence of a hormone produced by the placenta known as hCG
  • Home test kits test for the presence of hCG in urine
  • Doctors may perform a blood test to determine both the presence and amount of hCG giving an indication of how far along the pregnancy has progressed
  • If used improperly or too early, home pregnancy tests can offer false positive or false negative results
  • Some medical conditions and medications can also affect the accuracy of urine based testing
  • Home pregnancy tests are 100% safe, so when in doubt, test

What is a Pregnancy Test?

Pregnancy testing has changed markedly over the years, from injecting small animals with a woman’s urine to see if the hormones in the urine would cause the animal to ovulate or produce sperm, to urine testing that didn’t involve any animal involvement. No matter the form the testing took, these tests invariably looked for the same thing - evidence of hormones made during pregnancy. 

Modern tests examine the blood or urine of a woman to determine pregnancy by detecting the presence of a substance known as Human Chorionic Gonadotropin (hCG), a hormone produced by the placenta. Both home testing kits and tests performed by a doctor look for this hormone, with the major difference being that home test kits only test urine, but doctors may opt for a far more accurate and detailed blood test to not only detect pregnancy but how far along the pregnancy may be.  

How Does a Pregnancy Test Work?

Human Chorionic Gonadotropin (hCG) starts being produced when an egg is implanted in the uterus and the levels of the hormone increase exponentially for the first seven to 10 weeks of pregnancy, making this the optimal window for testing. After 10-12 weeks of pregnancy the levels of hCG plateau and then begin to fall.

There are two major types of pregnancy tests performed: urine and blood.

  1. Urine Tests

    Urine pregnancy tests are qualitative, meaning that they only test for the presence or absence of hCG, giving a yes or no result. These tests utilise a strip of material with a plasma membrane housing antibodies that can bind to hCG and dyes that react to this binding to give a visual indication of the presence of the hormone.

    Both at home testing kits and urine tests that a doctor may use work in this fashion but the method of testing can vary from manufacturer to manufacturer, with some tests needing to be dipped into a container of urine and others that can be held in the flow of urine. No matter the specific method required, urine testing is generally very fast and can show a result in only a few minutes.Home testing kits are readily available at chemist shops as well as a number of supermarket chains. Many home pregnancy test kits recommend testing the first urine of the morning as it is more concentrated and can give a better indication of the presence of pregnancy hormones.
  2. Blood Test

    Although blood tests also look for the presence of hCG to determine pregnancy, blood pregnancy tests are generally more accurate and can be used earlier. Blood tests can detect the presence of hCG by around 11-14 days of suspected pregnancy. Blood pregnancy tests are performed by a doctor who will take a blood sample and send it to a lab for testing. This testing usually takes a few days.

There are two kinds of blood pregnancy tests that can be performed:

  • Qualitative hCG Blood Testing - much like urine testing this type of blood test only looks for the presence of hCG in the blood, giving a yes or no result. Blood tests are generally considered to be more accurate than urine tests.
  • Quantitative hCG Blood Testing - this form of test not only looks for the presence of hCG but the hCG levels present, giving an indication not only of pregnancy but can also give an indication of how far along the pregnancy has progressed.  The easiest way to find and book an appointment with a doctor online for a pregnancy test is with MyHealth1st. 

When Should I Have a Pregnancy Test?

While the urge to get a pregnancy test as soon as you have missed a period may be strong, there are a number of factors, such as stress, diet, medication and exercise, that can cause an irregular period so it is advised that people wait for a week after a missed period to be tested. Not only can this delay give a more accurate result due to rising levels of hCG, it can also help rule out an irregular cycle.

If you don’t want to wait for a missed period there are a number of other indicators of a possible pregnancy, including:

  • Cramps - implantation of an egg can lead to cramps much like those some women experience during menstruation. If you experience cramps outside of your usual cycle, or experience cramps when you would usually have a period but there is no discharge you may want to take a pregnancy test.
  • Fatigue and Nausea - early pregnancy can often leave a woman feeling out of sorts, with fatigue or exhaustion, nausea, the need to urinate frequently and food aversions being common. If your body starts feeling different without being “sick”. You may want to have a pregnancy test. 
  • Sore of Tender breasts - Many women experience soreness or tenderness in their breasts in the days leading up to menstruation, but this discomfort can also be a sign of pregnancy. Higher levels of estrogen and progesterone due to pregnancy increase the blood flow to the breasts making them tender.

Are Home Pregnancy Tests Accurate?

Home pregnancy testing kits are entirely safe, so there’s no reason not to take a test if you believe you may be pregnant. When in doubt, test, but make sure to follow the instructions for the test kit so as to avoid potential false positive or false negative results.

Testing kits frequently market themselves as being 99% accurate, but these numbers are indicative of ideal circumstances. In reality there are a number of factors that can lead to false results, including:

  • Faulty Test Kits - faulty or broken test kits or those past their use by dates can deliver false positive results. 
  • Contamination - if a test requires that urine be collected in a cup, the presence of detergent residue or other contaminants can affect the accuracy of a pregnancy test.
  • Medication - some medications including antihistamines, anticonvulsants, tranquilisers, fertility drugs and diuretics can cause false positive results
  • Medical Condition - a number of medical conditions can affect the accuracy of tests or cause the production of hCG without pregnancy. These conditions include kidney damage (leading to protein in the urine), cystitis (causing blood in the urine) or ovarian tumours (producing hCG).
  • Testing Too Early - taking a test too early can mean that the levels of hCG in the body isn’t high enough to be detected leading to a false negative result. 
  • Improper Timing - not leaving a test in urine long enough can lead to false negative results. 
  • Dilution - drinking a lot of fluids may make you need to urinate, but it can also dilute the urine making the levels of hCG undetectable by a home test, giving a false negative result.

If you’ve taken a home test and the result is positive, then you should visit a doctor to confirm the test and talk about your options and care going forward. If you’ve taken a home test and the results came back negative but your instincts tell you otherwise, trust your instincts and see your doctor for a blood pregnancy test. 

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