No contraceptive method is 100% effective, so it is always advisable to take a pregnancy test if you think that you might be pregnant, regardless of the type of contraception you are currently using or have used in the past.
Hormonal methods of contraception, such as the contraceptive pill, contraceptive implants and injections, contain the hormones oestrogen and progestogen. They work by changing a woman's hormone balance.
However, these hormones will not affect the result of a pregnancy test because they are not used to measure whether or not you are pregnant.
How pregnancy tests work
A pregnancy test only reacts to the pregnancy hormone human chorionic gonadotropin (HCG). However, if you are pregnant, this hormone will not be present in your urine or blood until 13 to 16 days after ovulation (the release of an egg), which is around the time that you would normally get your period. Until this time has passed it will not be possible to see a positive result in a pregnancy test.
Urine tests require a certain level of HCG to be present in order to indicate a positive pregnancy result. Blood tests are more sensitive because they can detect a smaller amount of HCG, which means that pregnancies can be picked up earlier, usually between six to eight days after ovulation.
You can have a blood test at your GP surgery, but it is recommended that you take a home urine test before booking an appointment.
Human chorionic gonadotropin is sometimes used in fertility treatment, which could cause a false positive result (where the result shows as positive but is actually negative). You should wait 14 days after having fertility treatment before taking a pregnancy test.
A negative result
If you get a negative result after taking your pregnancy test, it can mean several different things. Firstly, it can mean that you are not pregnant, or that you have taken the test too early. Taking a test early is easily done because it can be difficult to work out the exact day that you began ovulating.
If you think that you may have taken the test too early, wait a few days before taking a second test or alternatively see your GP for advice and possibly a blood test.
Negative results may also be due to the fact you have timed the test wrongly. For instance, if you collect a sample of your urine for testing and have not conducted the test within 15 minutes, it may affect the result. Home pregnancy tests can vary, so always read the instructions carefully before you do the test.
Drinking too much fluid before a test can also lead to your urine being diluted, which can affect the levels of HCG in your sample. If you are in any doubt about your pregnancy test result, you should make an appointment with your GP.
Article provided by NHS Choices