A pregnancy test can let you know if you are pregnant. Here are answers to some common questions about home pregnancy tests.
On this page you will find information on:
- What is a home pregnancy test?
- How does a pregnancy test work?
- How accurate are home pregnancy test?
- When is the best time to do the test?
- Are all pregnancy tests the same?
- Can medicines interfere with test results?
- What happens next?
What is a home pregnancy test?
A home pregnancy test is a simple test using a small sample of urine (pee or mimi) to find out if you are pregnant. These tests can give a quick result and you can do the test in private. Home pregnancy tests can be bought from a pharmacy or over the internet and you can also get them from Family Planning centres.
How does a pregnancy test work?
All pregnancy tests detect the hormone human chorionic gonadotrophin (hCG), a hormone that is present only when you are pregnant. It starts to be produced after fertilisation and implantation of the embryo onto the lining of your womb during week 2 of pregnancy. This hormone enters your bloodstream and urine.
Most pregnancy tests come in a box with 1 or 2 long sticks. Generally you pee directly on the stick, or first in a jar and then briefly dip the stick in. The result appears on the stick after a few minutes. The current brand found at your GP or Family Planning centre normally uses a suction tube to transfer a small amount of urine into the testing window.
Results may show up after a few minutes as 2 lines for pregnant and 1 line for not pregnant, a colour or a symbol such as a '+' or '-' sign. It's important to know what a positive or negative result looks like for the brand of test you are using. All tests are slightly different, so always check the instructions.
How accurate are home pregnancy tests?
Home pregnancy test manufacturers generally claim the tests are about 99% accurate. However, this number is taken from research in the lab, not real life outside the test tube. The real life accuracy is lower, especially in early pregnancy.
When trying to detect pregnancy on the first day of a missed period, up to 46% of woman will have a falsely negative result, depending on the brand and technique of the user. The accuracy is much higher if you wait longer and repeat the test after a week.
A home test’s accuracy depends on:
- how closely you follow the instructions
- when you ovulate and how soon the egg implants
- how soon after you become pregnant you take the test
- the accuracy of the pregnancy test brand.
A positive test result is almost certainly correct. A negative result is less reliable. If you have a negative test but still think you might be pregnant, repeat the test 1 week after your missed period or talk to your healthcare provider.
Could a positive result be wrong?
Although uncommon, it's possible to get a positive result from a home pregnancy test when you're not actually pregnant. This is known as a false positive.
- Pregnancy loss: A false positive might happen if you had a pregnancy loss soon after the fertilised egg attached to your uterine lining.
- Technique of doing the test: Incorrectly doing or interpreting the test can cause false positives.
- Ectopic pregnancy: The embryo has implanted outside your womb.
- Rare causes: These include some fertility treatments, hormone secretion from a tumour or from your pituitary gland, and other causes.
Could a negative result be wrong?
It's possible to get a negative result from a home pregnancy test when you're actually pregnant. This is known as a false negative. You might get a false negative if you do either of the following:
- Taking the test too early. This is the most common cause for a false negative result. The earlier after a missed period that you take a home pregnancy test, the harder it is for the test to detect hCG. For the most accurate results, repeat the test 1 week after a missed period. Many kits provide a second test for this purpose. If you can't wait that long, ask your healthcare provider for a blood test. The blood test is positive earlier than the urine test.
- Checking test results too soon. Give the test time (usually a few minutes) to work. Set a timer according to the package instructions to be sure you've waited for the right amount of time.
- Drinking too much water before the test. Drinking a lot of water can dilute your pee and make it harder for the test to pick up the hormone.
When is the best time to do the test?
Most pregnancy tests can be used from the first day of a missed period. If you don't know when your next period is due, do the test at least 21 days after you last had unprotected sex.
- During early pregnancy, the hCG concentration increases rapidly – doubling every 2 to 3 days. The earlier you take the home pregnancy test, the harder it might be for the test to detect hCG.
- Timing of your ovulation might vary from month to month, and the fertilised egg can implant in the uterus at different times. This affects when hCG can be detected.
- If your periods are irregular, it can be difficult to know exactly when your period is due.
Your hCG levels are most concentrated in the morning, so it's best to do the test in the morning, but you can do a pregnancy test any time of the day.
Are all pregnancy tests the same?
All home pregnancy tests look for hCG in urine. For most of them you need to pee on the stick and the result appears on the stick after a few minutes.
Some home pregnancy tests are more sensitive than others, so the amount of hCG needed to show a positive test result is lower in some tests. The tests may also vary in the length of time you need for results to show and how the results are displayed, eg, some tests sticks change colour or display a symbol to indicate whether you are pregnant.
Always check the test's expiry date and read the instructions carefully before you take the test.
Can medicines interfere with test results?
Fertility medicines that contain hCG might interfere with home pregnancy test results. However, most medicines, including antibiotics and birth control pills, don't affect the accuracy of home pregnancy tests.
What happens next?
Based on your test results, consider taking the following steps:
Your home pregnancy test is positive, or you've taken a few home pregnancy tests and had mixed results: Make an appointment with your healthcare provider. You might need a blood test to confirm your pregnancy. The sooner your pregnancy is confirmed, the sooner you can begin prenatal care. Ultrasound scans are not routine in New Zealand for detection of pregnancy, but one can be done from around 5–7 weeks if needed.
Your home pregnancy test is negative: If your period doesn't begin, repeat the test in a few days or 1 week, especially if you took the test before or shortly after a missed period.
You have negative test results, but your period doesn't begin and you think you might be pregnant: Check with your healthcare provider. Many factors can lead to missed periods (amenorrhea), including thyroid disorders, low body weight, problems with your ovaries, excessive exercise and stress. If you're not pregnant, your healthcare provider can help you with any other health concerns.
- Detecting pregnancy and ovulation with home test kits US Pharmacist
Angela is a pharmacist in the Quality Use of Medicines Team at Waitematā District Health Board. She has experience in hospital pharmacy in New Zealand and in the UK, and was previously a medical writer for Elsevier in The Netherlands. Angela is interested in promoting the safe use of medicines, particularly high-risk medicines.