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How Common Is A False Positive Pregnancy Test And What Causes It?

Home Pregnancy Tests

The best way to get accurate results when using a home pregnancy test is to wait until your period late. At that time, the level of hCG in your sytem will be large enough that all of the pregnancy tests on the market will be able to detect it, producing a positive pregnancy test result.

Most women, who are trying to get pregnant, prefer to find out as soon as possible. That “two week wait” becomes unbearable. There are many home pregnancy tests on the market that claim you can test much earlier, usually 4 or 5 days before your period is due. These early test pregnancy tests are typically about 97% accurate, when all the instructions are follow exactly and you read the results within the time frame listed in the instructions.

What Can Cause False Positive Pregnancy Test Results?

A truly “false positive” test result, due to a faulty home pregnancy test is fairly rare. A positive result which is not repeatable, with subsequent home pregnancy tests, is usually because of one of these reasons: Chemical pregnancy, missed reaction time, chemical interference or an evaporation line on the pregnancy test.

Chemical Pregnancy

As unfortunate as it is, about 25% of all pregnancies end up being a chemical pregnancy, also known as an early miscarriage. A chemical pregnancy happens when a pregnancy is lost shortly after implantation. With the advancement in sensitivity of the home pregnancy tests on the market today, women are able to detect pregnancy in the very, very early stages. This is the downside of the super sensitive pregnancy tests – the joy of discovering pregnancy very early on, followed by crushing heartbreak if an early miscarrige occurs.

Missed Reaction Time

You really need to follow the instructions included with your home pregnancy test exactly. The manufacturer lets you know how long you should allow the test to sit before interpreting the results. If you let the test sit for longer than the instructions state, any results you interpret need to be considered invalid. Reading after the recommended time frame has passed can show results that you will misinterpret as a positive result.

Chemical Interference

If you are undergoing fertility treatments and have had a hCG (Human Chorionic Gonadotropin) trigger shot, you will need to make sure the shot has had enough time to metabolize through and clear your system. Fertility doctors suggest waiting about 14 days for your system to be clear of the hCG before taking a pregnancy test. Testing before this time may very well likely produce a positive result when you are not truly pregnant.

Evaporation Lines

Some pregnancy tests can leave an evaporation line on the pregnancy test, as the urine crosses the result area. The evaporation line is usually a very faint, grayish line in the results area of the pregnancy test. The color of the line is not what the test instructions say should be there, for a positive result, but many women, will erroneously interpret the line as a positive result.

If you feel you are having early pregnancy symptoms and not getting the results you think you should be from your home pregnancy tests, you should contact your doctor for a blood pregnancy test.

  • cole

    i took two clear blue digital tests 20 min apart. the first one was positive the second one was negative. is it possible its a false positive?

  • http://www.earthsmagic.com/ Vickie B.

    It isn’t something that happens often. Maybe your level of hcg is low right now. If you used two different batches of urine to test, the first one could have been more concentrated as it had longer to build up in your system. If you urinated again 20 minutes later to test, it could be the hcg just didn’t have enough time to build up again. This would be especially true if you haven’t missed your period yet.

  • terri

    My husband and I have been together for 8 years and have been trying to conceive… I have thyroid disease and my cycle has never been accurate. I’m 90 days in which has happened before but this prompted me to take a pregnancy test (clear blue digital) yesterday morning at 4 am and it was positive and then I took another at 3 pm and positive… Now I worry that they are both wrong just cause of all the issues I have had in the past.. What is your opinions? I am making a dr apt tomorrow to either confirm or crush this feeling… Confused and Nervous

  • http://www.earthsmagic.com/ Vickie B.

    Hi Terri! Sorry it took so long to reply. :( Did you get confirmation from the doctor?

  • Jessica

    I took 5 hpt total. First was 3 days before missed period with clear blue digital at 4:30 am, said pregnant. Tested again later that same day results, not pregnant. Decided to wait until the day of my period took 2 more tests this time a store brand both negative results. Now I am 4 days late and toik another clear blue digital hpt says not pregnant. Should I trust the very first test that showed pregnant? Confused…

  • http://www.earthsmagic.com/ Vickie B.

    If you tested with first morning urine on the second clear blue, then unfortunately, it sounds like you may have had an early miscarriage. If you didn’t take it with the first morning urine, try again, limiting fluid intake and holding your urine for as long as possible.

  • Jessica

    The second clear blue was not first morning urine. The first store brand was but I didn’t pee for 5 seconds on the stick. I’m now 5 days late for my period. Still no cramping or any signs of it coming.

  • http://www.earthsmagic.com/ Vickie B.

    I would suggest that you try again with another test but make it the FMU. If you are pregnant, that should show up positive.

  • Sara

    Hi, I was sterilised 5 years ago and i’m 4 days late (im never late), so I took a test (not morning) said “preganant, i waited til I needed to pee again and took the 2nd to make sure and it came back ” not pregnant”. So I’ve done another this morning (FMU) and said “not pregnant” again…. sooo confused!!

  • http://www.earthsmagic.com/ Vickie B.

    False positives are pretty uncommon with the popular pregnancy test brands. The fact that you were sterilized makes a positive even more odd. If your period doesn’t arrive soon, I’d suggest a visit to the doctor and explain what occurred. Sorry I couldn’t shed any sort of light on this issue for you.

  • sara

    Yeah, i have spoken to an out of hours doctor, I’am going for blood tests tomorrow morning (monday) to try and get some answers. I still have no sign of a period but I’am cramping and feeling sick now, so don’t know whether thats a good sign or not

  • http://www.earthsmagic.com/ Vickie B.

    Good luck! Hope you get some answers that feel good to you!

  • Jmcat01

    Ok so I took a clear blue pregnancy test last night and it said I was pregnant then I took on this morning and it said not pregnant…idk what go think

  • http://www.babyhopes.com/ Vickie B.

    It could be that you are testing a tad too early. Wait a day and try again. Try and make sure you are not drinking much liquid for several hours before testing so the hcg has time to build up in your urine.

  • Gabby

    I am 10 days late on my period and took a test which to my surprise came back positive, this is because I have PCOS and believed it would be hard to get pregnant. However, after having a positive I took another one to show my partner which came back negative. All together I have taken 6 pregnancy tests and had 5 negative and 1 positive. Why is this, I’m confused and don’t know what to do.

  • Mil

    I did a hpt this morning it was a faint positive I went to the doctors they’re test was negative Iv has a blood test but got to wait for results. I done anothe test when I got back another faint positive help??!!

  • http://www.babyhopes.com/ Vickie B.

    It could be that you have very low levels of hcg. By now, the positive should be much darker if you are pregnant. It could also be that you had a very early miscarriage. If that is the case the pregnancy tests would be showing a negative by now.

  • Sarah

    Hi, I went to my Ob-Gyn last week and she made an ultrasound for me. She said that I am pregnant but no fetus/embryo was present. There’s no yolk sac, either. But it said that it’s 6 weeks already, and as I am typing this it’s already 7 weeks. I don’t feel any symptoms that I am pregnant, except for my head aches sometimes and feel tired easily. Is it false pregnancy? Thanks

  • http://www.babyhopes.com/ Vickie B.

    You will need to contact your doctor about this to find out exactly what she means and what it means for you. If there isn’t an embryo present, I would think that it isn’t a pregnancy, unless what actually an early miscarriage that hasn’t completed yet.

  • Krista

    I am 42 yrs old and I just finished chemotherapy for NHL in Feb. , which forced me into menopause. The oncologist told me that I would resume my normal cycle about 2 months after completion of chemo. I got 2 sort-of periods that stopped about 2 months ago . I started getting the hot flashes with no period after that and went for my yearly at the gyn. I was just informed that I MIGHT be pregnant! They said they tested my urine and there was a line in the positive, but was faint…I have to wait now for blood results. What are the chances?!! What does a faint line mean??

  • Ashley

    So I had my period on July 5th and August 6th. It lasted till the 10-11th. I had unprotected sex and my bf didnt pull out on the 14th. Usually I know when my period is coming bc I get massive cramps and I get emotional & grumpy. It’s now the 11th & no signs of my period. Which I expected to arrive around the same time. I took an expired test and said neg so then I bought a new one and took it at night and said neg. yesterday the 10th I drank water all day and up till midnight bc it’s all I rly drink. Today the 11th at 5:30 I took another and said neg. is it possible the water is diluting the test for a false neg? I don’t understand how neg tests but no period. I haven’t changed anything to delay my period. And I find it crazy that it would be “delayed” the month after unprotected sex. I’m not sure if I should wait and take another test, wait for my period, or schedule a doc appt for a blood test. HELP! I’m just not sure if I would be considered far enough based on when I had sex to determine the test positive. Feedback plz. ( & no rude comments, yes I am old enough for a child. & two I do want to be pregnant. Just not sure why neither are appearing )

  • Emily32013

    I had my tubes tied in October of 2012 and I recently missed a period which could easily be due to stress but I took a test Bc I have been nauseous, dizzy And just didn’t feel right. It was negative at first then it showed a faded line positive after the time . Ok so I repeated the test the next day and the same thing happened. I did have a two day period though. Could I be pregnant? There’s no way right??

  • John Glen Garrett

    ????

  • Adebisi Ayannusi

    It is becoming a common occurrence
    when you do a blood pregnancy test and giving a positive result but the truth is that the patient is not pregnant. Pelvic Scan was carried and showed no evidence of cyesis. It comes worrisome as one of the patient is still intact. Much as i know that a choro – carcinoma can result in false positive, does it mean that these young women are in that group?

  • Amanda

    I’m 4 days late so I took 3 home test and they were all positive. I go to the doctor and it was negative! I don’t know what to think.

  • Kelly

    I am a week late. I took a clear blue easy test. The line is faint. But it’s positive.. Then I took the dollar store pregnancy test. The line is still faint, but it says positive! Am I really pregnant?! :)


Last modified: February 14, 2014


The information provided here should not be considered medical advice. It is based on the average experience of women trying to conceive and may not be what you may be experiencing. It's not meant to be a replacement for any advice you may receive from your doctor. If you have any concerns about your cycle or our ability to get pregnant, we advise you to contact your doctor.