You can do a pregnancy test from the first day of your missed period.
If you have regular periods, you’ll probably know when this is.
If you’re not sure when your next period is due, you can do a pregnancy test 21 days (three weeks) after you last had unprotected sex.
See Further information below for links to more about how your menstrual cycle works.
Some very sensitive tests can be used even before you miss a period. Check the packaging of your test to find out when it can be used.
What’s being tested?
Pregnancy tests check for the presence of the pregnancy hormone, human chorionic gonadotrophin (HCG) in your urine.
After you conceive (when the sperm fertilises the egg), your body begins to produce HCG. Around two weeks after you conceive, there’s enough HCG for most pregnancy tests to detect it.
When did I conceive?
To conceive, an egg needs to be released from your ovaries (ovulation). Ovulation usually happens about 14 days after the first day of your last period.
An egg lives for about 12-24 hours after it’s released. For you to get pregnant, a sperm must fertilise the egg within this time.
Sperm can live for up to seven days inside your body. This means, if you have sex up to seven days before you ovulate, or within a day or so of ovulating, you could conceive.
If you are trying to get pregnant, guidance from NICE advises that, for the best chance of success, you should have sex every two to three days throughout the month. You don’t need to time it to coincide with the days when you ovulate.
Around seven days after you conceive, HCG might be noticeable in your urine in very low levels, though this varies from woman to woman. Most tests need a higher level of HCG to give a positive result.
Results of the test
A positive test result is almost certainly correct. If you do a pregnancy test on the first day of your missed period, and it’s positive, it’s probably about two weeks since you conceived. You can use the pregnancy due date calculator to work out when your baby is due.
A negative result is less reliable. If you still think you’re pregnant after a negative result, wait a week and try again or see your GP.
For more information, see How accurate are home pregnancy tests?
Testing after a miscarriage, abortion or ectopic pregnancy
After you’ve had a miscarriage, abortion or an ectopic pregnancy, it’s possible that a pregnancy test may still test positive for a few weeks, even though you’re no longer pregnant. This is because HCG may still be present in your body.
You may want to get advice about this from your GP.