What is a false negative pregnancy test?

A negative pregnancy test result does not always mean you are not pregnant.

what is a false negative pregnancy test
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If you miss your period, you may be wondering if you are pregnant. But what happens if your breasts are tender, you feel nauseous and you are convinced that you are pregnant but the home test result is negative?

Sometimes you can get a false negative on your pregnancy test – meaning you are pregnant, but the test says otherwise. On the flip side, you can also miss your period when you’re not pregnant. Dr Louise Wiseman explains what a false negative pregnancy test means:

What is a false negative pregnancy test?

Home pregnancy tests show a positive result or a negative result. A positive result indicates a pregnancy but a negative result does not always mean you are not pregnant and might just mean it’s too early to say.

A false negative pregnancy means the test shows a negative result when you have actually conceived so you might go on to have a positive pregnancy test later in that cycle.

How do home pregnancy tests work?

All pregnancy tests, whether in the lab or at home, work by detecting a hormone that is specifically produced in pregnancy called Human Chorionic Gonadotrophin or hCG.

When a human egg is fertilised, it attaches to the lining of the womb. This process is called implantation. The placenta embeds into the womb and its blood vessels act like a channel between the developing fetus and the mother. This is how the baby receives nourishment and how waste products are removed.

A negative pregnancy test result does not always mean you are not pregnant and might just mean it’s too early to say.

The placenta releases the hormone hCG (even in small amounts before implantation) which causes many of the changes that occur in pregnancy, including telling the body not to have a period. At the same time the hCG enters the mother’s bloodstream and urine and can be detected by testing urine or blood.

hCG levels increase rapidly after implantation, doubling every two or three days and peaking at about six weeks of pregnancy. This six weeks is dated from the first day of the last period. It makes sense that a test is more likely to be positive, for example when you are six weeks pregnant, (about two weeks after your normal period date) than when you are just about to miss a period, as the hormone levels have had time to climb.

what is a false negative pregnancy test
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How to take a home pregnancy test

Urine is tested in home kits as it is much easier to do than a blood test. A home pregnancy test requires either peeing onto a stick or placing a dipstick into wee that you have collected. A home pregnancy test usually involves the following:

  1. Carefully follow the instructions specific to that pregnancy test.
  2. There will be a required time to wait until reading the test result in the test window. This tends to be one to three minutes.
  3. The results will be read as a plus or minus sign, one or two lines or the words pregnant or not pregnant.
  4. Some tests have a control window to check against so you can see the test is working.
  5. If the control window does not do what it says on the instructions, you may need to repeat the test with a new one as it simply has not worked.

    Types of home pregnancy tests

    Pregnancy tests are readily available without a prescription in pharmacies, online and some supermarkets. Pregnancy tests are also provided free of charge at community contraceptive clinics, sexual health clinics, Brook Centres for Under 25s and some young people’s services.

    All pregnancy tests are thought to be 99 per cent accurate if you test on the day your period is due. There is some variation in how sensitive the tests are when the hormone hCG is at very low levels at the beginning. For this reason, most doctors advise waiting until at least your period is due before testing. We are however all human and many people are naturally impatient so may prefer to test a little earlier.

    • Early pregnancy tests

    Some early tests claim to be able to test five days before the due period date. Levels of hCG may still be low so be prepared to repeat the test if it is negative, as it may simply be too low to detect yet.

    The more sensitive a test is the better it is at picking up low levels of hCG in the system. The sensitivity of a test will be clearly displayed on the box. This will be the hormone level of hCG that it detects in milli-international units per millilitre (mIU). A test that detects at a level of 25mIU will be more sensitive than one that detects at 50mIU.

    • Digital pregnancy tests

    Some digital tests are reported to be more accurate than non-digital tests. The result on a non-digital test involves looking for a coloured line. A digital test will have a countdown to when it is ready and will display the words pregnant or not pregnant. Many women find this easier to use, but it is simply a question of personal choice. Speak to your pharmacist about choosing the correct test for you.

    Reasons for a false negative pregnancy test

    Your pregnancy test may come back false negative for the following reasons:

    • You have tested too early in your cycle.
    • You have not waited the correct time for that test (set a timer as per the packet instructions).
    • The test is out of date.
    • You have drunk a lot of fluid and diluted any hCG in the urine to an undetectable amount.
    • Time of the day. You can test at any time of the day with most tests, but some stipulate an early morning sample. This is because it will have a relatively higher level of hCG as urine becomes more concentrated overnight.
    • If you have irregular periods or have just stopped the contraceptive pill to conceive, you may have ovulated a little later than you thought. You may only just have conceived so it will just take longer for the test results to show.

      Understanding your ovulation window can be helped by online ovulation calculators that look at recent cycles or using ovulation urine tests. If your cycles vary in length and you have a negative test now you may need to do some maths. Work out your longest cycle and use that to calculate your period due date and test at that point if you still have not had a period.

      Reasons for a negative test if you're not pregnant

      You can miss your periods or have delayed cycles for a number of reasons and this often settles on its own. If you have not had a period for three months and do not know the reason, then consult your doctor. There can be a number of reasons for an irregular period including the following:

      • Thyroid disorders
      • Polycystic ovarian syndrome
      • Extreme low body weight
      • Drastic weight change
      • Stress
      • Excessive caffeine
      • Excessive exercise
      • Lifestyle changes - eg starting night shift work
      • Breastfeeding can affect your cycle
      • Change of hormonal contraception method
      • Certain medications - eg allergy, blood pressure medications, antipsychotics, chemotherapy, antidepressants.
      • In less than three per cent of ectopic pregnancies there is a negative pregnancy test despite there being a pregnancy.

        What do you do if you suspect you are pregnant

        If you suspect you are pregnant but have had a negative home pregnancy test result the most sensible thing to do is to be patient, although this is not always easy. Wait three days or even a week for any hormone levels to rise and if your period has not started during this time, take another test.

        It can be very tempting to buy a lot of pregnancy tests and keep taking them, but waiting a little time to do a test with an accurate result may be better than riding an emotional rollercoaster. If you have concerns, then talk to your doctor or healthcare provider.

        Last updated: 03-01-2021

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