What do they do?
Pregnancy tests measure the amount of a hormone called human chorionic gonadotrophin, otherwise known as hCG or beta-hCG, in the body. The hormone is secreted by cells of what will become the placenta as soon as the embroyo attaches itself to the wall of the uterus. It is not found in women who are not pregnant, and can be detected 8 days after fertilisation. It allows the body to maintain ovulary secretions and prevent periods, so that the egg remains attached to the wall of the uterus. The amount of hCG present in the system doubles every 2-3 days, reaches its highest level at around the 12th week of pregnancy, and then falls. It remains detectable in the blood and urine throughout pregnancy.
Different pregnancy tests
- Home urine pregnancy tests
These are the most popular and are sold in pharmacies. They detect the presence of the hCG hormone in urine using antibodies which react positively or negatively and display a certain colour (a blue line or little cross). Urine pregnancy tests are 99% reliable. It's extremely rare for a test to come up positive if you are not pregnant. However, a urine test can come up negative if a pregnancy is underway if you take the test too early or if the test is past its use-by date.
- Blood pregnancy tests
There are two types of lab blood tests that your GP can carry out. A qualitative test is simply either positive or negative, while a precise quantitative test analyses the exact quantity of the pregnancy hormone in the body and can estimate the date of fertilisation. Blood pregnancy tests are 100% accurate and they are the most reliable way to find out for certain if you are pregnant or not. They will only be carried out though if you have any problems or bleeding in early pregnancy.
- Saliva pregnancy tests
These are not very well known or widely used but are very practical. They measure the amount of the hCG hormone present in the saliva and are 98% accurate.
When should you take a urine test?
It's vital to take a urine test at the right time if you want a reliable result. Take it 10 days after fertilisation (by which time the level of hCG in your blood will be high enough to be detected by a urine test if you are pregnant); or otherwise after your period is due (wait for at least 3 days). If you get a negative result, take another test a week later. If you can, take the test in the morning when your urine is more concentrated and will contain higher quantities of hCG (unless you've drunk a lot the night before). But you can take a pregnancy test at any time of the day as long as you haven't drunk a lot of fluid, so that your hCG won't be too diluted.
How to take a urine test
Hold the test under your stream of urine briefly to let a few drops run onto the swab, or take a sample of urine and dip the swab in it. In a few minutes, a sign will appear showing whether the test is positive or negative.
A false positive result (you are not pregnant but the test is positive) can show if you are on some types of medication (such as antipsychotic drugs), if there is blood present in your urine, or if you have used a detergent to clean the container used to take a urine sample.
A false negative result (you are pregnant but the test is negative) can show if you have a urinary tract infection, if you have drunk lots of fluid prior to your test, or if the fetus is growing outside the womb (an ectopic pregnancy).
How much does one cost?
- A urine test costs between £5 and £15.
- You can ask your chemist or Family Planning Clinic for a test, although this may not be instant.
- Saliva ovulation tests are available on the internet, but actual pregnancy saliva tests are not in wide use and are harder to find.