Traces of human chorionic gonadotrophin (hCG) can be found in your urine from three to four days after the fertilised egg implants in your womb (uterus) and the amount builds up each day.
A pregnancy test can only detect hCG and give a positive result when there's enough of the hormone in your system. This means you can take most tests from the day you miss your period.
If your periods were irregular you might not be sure when this. You can take the test anyway but if it is negative, repeat it after a week to check again.
Home pregnancy tests
You can buy a home pregnancy test cheaply from a chemist or supermarket and find out if you are pregnant in minutes. You have to hold the end of the pregnancy test in a stream of urine, or dip it in urine you collect in a container.
They are very reliable, although it is important to follow the instructions on the packet carefully. The symbol showing that you are or are not pregnant can be different, so check before you start what mark or symbol will appear if the test is positive.
If the pregnancy test says you are pregnant it is almost certain that you are.
Pregnancy tests at your doctor or local chemist
These tests are either free or you may be charged a small fee.
You’ll need to take a sample of your urine in a clean, soap-free container. You can get pots to put your sample in from your GP’s surgery. The best time to collect a urine sample is first thing in the morning. This is because levels of hCG will have built up overnight.
You may have to wait up to a few days for the result, or if your urine sample is tested on the spot, you'll probably be given the result after just a few minutes.
Your pregnancy test results
A positive result is very likely to be accurate but a negative result might be misleading, it depends on your timing.
Some women have a negative result and test again a week later to find that it is positive. This is because the level of pregnancy hormones gradually builds up over time. If your levels are too low to detect the test might be negative even though you are actually pregnant.
If you do a home test very early on and have a negative result, do another test in two to three days' time if you still haven't had your period.
If you continue to get negative results and still don't have a period, talk to your doctor.
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Pregnancy tests, hCG levels and miscarriage
hCG is the hormone that is produced by the embryo after implantation. A raised level of this hormone shows that there is or has been a pregnancy.
The level of hCG in a pregnancy usually doubles every 2-3 days in early pregnancy, then doubles every 3-4 days later on until about week 14 -16 of pregnancy.
If a miscarriage has happened then you can still have positive pregnancy tests and raised hCG levels for up to 2 weeks afterwards as the levels take some time to come back down.
Are more expensive digital pregnancy tests better?
If you've been trying for a pregnancy for a while you'll find that pregnancy tests become an expensive part of your life. You might feel under pressure to choose more expensive branded or even digital pregnancy tests. Be reassured, the cheaper own-brand tests bought from a reputable supermarket or pharmacy work as well as these.
Digital tests can be more tempting because they give you the 'window' of how pregnant you are. However, they are not more effective at telling you whether or not you are pregnant, and, as they have a an extra digital component, this is an extra element that may fail.