How pregnancy tests work
Well, you’ve waited and waited, your body feels like it’s changing already (or perhaps it doesn’t) and you are dying to find out whether that sneaking suspicion that you might be pregnant is actually correct.
A lot of women planning to become pregnant keep a stash of home pregnancy tests – and that’s not a bad idea, these days they are pretty accurate when used correctly.
Pregnancy tests return a result based on the level of the hormone hCG (human chorionic gonadotropin) which is released into the bloodstream soon after conception, initially at very low levels but increasing rapidly till about twelve weeks into the pregnancy.
While research has shown that around five percent of women will return a positive result from a home test as early as eight days into the pregnancy, most experts recommend waiting until at least 14 days past the likely conception date.
If you are very keen to get an early result, discuss your options with your doctor; it is possible to confirm an early positive result from a urine sample with an early blood test, administered by your doctor, from around ten to twelve days after conception.
Testing too early may give a negative result even though you may still be pregnant. And to prevent early hopes getting too high, an early positive result should be checked again around 14 days after conception is thought to have occurred to ensure that the result is accurate.
Information on home pregnancy tests
There’s a big range of home pregnancy tests on the market which test for levels of the pregnancy hormone hCG in the urine. But in general, if you follow all the instructions, the tests are around 97 percent accurate.
The more sensitive tests claim to detect pregnancy earlier than others – and are therefore more expensive. Tests will measure how much hCG is present and report it in mIU per millilitre. More sensitive tests will detect anything over 20 mIU per ml and report it as positive, giving a result from as early as eight days after conception; while most tests will detect around 50 to 100 mIU per ml before returning a positive result.
The best time to test is first thing in the morning, as hormone levels are likely not to have been diluted, although most tests these days will give results any time of day. Avoid drinking lots of water before testing so you don’t dilute the urine too much.
Before you take the test, make sure you read the instructions carefully – when you are anxious or excited it is easy to get mixed up, so take a deep breath and be as calm as possible.
Use a watch or clock with second hands if possible, so that you are definite about the time that has passed. You may even want to write down the time that the test was started if you are feeling very scatty. Make sure that you check the test result within the recommended time limits.
Most tests ask that you hold the pregnancy test stick into the urine mid-stream – a bit tricky and possibly messy. Some test veterans suggest bringing a container or a cup into the loo, collecting a little urine mid-stream and then dipping the test in to reduce the risk of missing.
The general rule on pregnancy tests is – one line (the control) means NO, you’re not pregnant; two lines mean YES – you are pregnant.
However, sometimes tests show an ‘evaporation line’ which can confuse the issue. Read the instructions if you’re not sure – and most instructions also have a customer support line that you can ring if you are still unsure.
Advice for blood tests
Pregnancy tests based on urine samples are so accurate these days that blood tests are rarely used to confirm pregnancy. Blood tests used by doctors are estimated to be more than 99 percent accurate and will test for the presence of hCG. Some tests will check the level of hCG, which may give an indication of how far along the pregnancy has progressed. A blood sample is generally sent to a pathology laboratory and results available in a day or two.
False Positives results
Usually, if a test says you are pregnant – you are, but occasionally, a ‘False positive’ result can occur. These can be heart-breaking and are rare, but may occur for a few reasons:
- ‘Chemical pregnancy’
This is a term used for a very early miscarriage. About half of all pregnancies are thought to end in miscarriage, usually shortly after conception. But if a test is taken very early, when hormone levels have started to climb, you may detect a positive hCG response and then go on to have a period.
- Not following directions
Make sure you check the pregnancy test within the specified reaction period; a test that has been sitting around too long may return a false positive result.
- Fertility treatments
Some fertility treatments contain the hCG hormone and may return a false positive if the hormone is still present.
False Negatives results
If you test too early, you may get a negative result even though you are pregnant; testing a few days later may return a different result, so do keep an eye on the changes in your body just in case. And in rare cases, a pregnancy test may not be working properly (although the test strip usually shows if this is the case) so you will need to re-test.
By Fran Molloy – journalist and mum of 4
Last Published* April, 2023
*Please note that the published date may not be the same as the date that the content was created and that information above may have changed since.