If you are trying to get pregnant or think that you may be pregnant, you may be wondering; When can I take a home pregnancy test? How soon will a home pregnancy test show as positive? Can you get a false positive on a home pregnancy test? Can you get a false negative on a home pregnancy test? How do pregnancy tests work anyway?
For answers to these burning questions about home pregnancy tests and more, keep reading.
Disclosure – I am not a doctor, just a Mum of 4 who has been through a number of fertility journeys and has taken a lot of pregnancy tests. For personalised advice, please see your own doctor.
Table Of Contents
How Do Home Pregnancy Tests Work?
Over-the-counter home pregnancy tests (HPTs) work by testing for the hormone “Human Chorionic Gonadotropin” or hCG in your urine. This hormone will only be present during pregnancy (or in some rare cases that we’ll discuss below).
Read the instructions on the specific test that you purchase, but all of the tests work in a similar way which is detailed below.
Pregnancy tests work by weeing on the absorbent end, (or if you don’t have an amazing aim and don’t want urine all over your hands, wee into a cup and dip the absorbent bit). The fancier, more expensive tests will have a plastic cap that you can then replace to cover the bit soaked in wee, other cheaper versions won’t.
One line or two? What are the lines for anyway?
The test will then usually display one line or two, (unless you’ve paid the extra to “go digital” which we’ll discuss in a second). The first of the two lines is the “control line”. If you cast your mind way back to your GCSE science lessons, you’ll remember that this control line simply indicates that the test has taken place. This line doesn’t show any form of positive or negative result, it merely indicates that you managed to aim right and wee on the correct bit (well done you).
The second line is the important one. The second line is ONLY present if hCG, the pregnancy hormone is detected. If this line is present, it means that the test is positive and you are pregnant. If the line is not there and only the control line is present, then the result is negative.
Going Digital
If you want a pregnancy test spelt out in black and white, a digital test could be a good option. Personally I found the traditional tests with lines were perfectly accurate and easy to read, but my husband didn’t seem to trust them. For this reason, I used to buy they normal line based tests for my own sake, then once I had a positive test I would then take a more expensive digital test to show my husband. A test with the word “pregnancy” on it seemed to be the only way to convince him.
The digital tests work in the same way, as in they look to detect hCG, but they simply display “pregnant” or “not pregnant” instead of showing lines.
Can you get a false negative?
Yes, if you take the test “too early” (and we’ll talk about timings in a second), you can get a false negative. Quite simply, if you have conceived, but not enough time has elapsed since implantation (which I will also cover below) for the levels of hCG to build up to be detected in your urine, then the result will show as negative.
Testing at the wrong time of day can also show as a false negative in the early days.
Mosts tests (again read the instructions on your own test, but generally speaking) work best if you test first thing in the morning with your first wee on the day.
Why is this? Simply because your wee is usually more concentrated at this time of day as it has been less diluted by all of those fluids that we are supposed to be drinking.
Can you get a false positive?
The short answer is – no. If hCG is present, then this hormone can only have been created by pregnancy. The exception to this is when you have experienced a chemical pregnancy.
What is a chemical pregnancy?
This horribly clinical term is used to describe a pregnancy that has been lost very early on, often around the time that your normal period would be due. In this case, the levels of hCG will diminish after the pregnancy is lost and subsequent pregnancy tests taken in the days and weeks to come would have the second line becoming fainter until it eventually disappeared.
There are 2 exceptions when pregnancy tests can show a false positive result
- Chemical pregnancies
- Where some cancers are present
How long should you wait before taking a home pregnancy test?
The official answer to this question according to the NHS website, is that the earliest you should take a pregnancy test is the first day of your missed period. The truth of the matter is that the answer is more complicated than that. Allow me to explain.
Why do they suggest waiting until the first day of a missed period to take a test?
This date is calculated based upon the “normal” cycle of a woman of a fertile age. See the video below that explains the menstrual cycle and what exactly goes on.
In a woman with a standard 28-day cycle, you could expect;
- Day 1 – period begins
- Day 14 – ovulation
- Days 14 – 28 “luteal phase” (the bit between ovulation and when your period is due).
- Day 28 – period is due
What is the two week wait?
The two-week-wait (or TWW as it is referred to when in online fertility communities and forums), refers the luteal phase, the bit between ovulation and when your period is due. This is the period where you are waiting after ovulation for the time when your period is due and you can take a pregnancy test and be reasonably confident that the results will be accurate.
Do I really have to wait 2 weeks to see a positive result?
The answer to this question is “it depends” upon a few factors, such as…
The Sensitivity Of The Pregnancy Test
Each test will have a different sensitivity level. hCG is measured in milli-international units per millilitre (mlU/ml). If you check the packaging of the test that you are purchasing, it will show a number before of mlU that the test will detect. The lower the number of mlU the test shows, the more sensitive the test and the earlier that the pregnancy test will be able to detect pregnancy.
For example: a 25mlU test will detect pregnancy and give a positive result before a 50mlU test.
In normal, healthy pregnancies, the hCG levels will double every 48-72 hours. The longer you can wait to take a test, the more likely it is that the test will give an accurate result. In some cases, a positive test will not show up until several days after a missed period. Often, in these cases, it is because the test being used has a higher mlU number and can only detect higher levels of hCG.
Ovulation dates are more relevant than period due dates
If you have been tracking your fertility and monitoring ovulation, the way I talked about in my fertility story, then you may find that you ovulated earlier, or indeed later than day 14. When taking a pregnancy test, it is more relevant how many days past ovulation (DPO) you are than when you expect your period.
So why do pregnancy tests talk about days before or after an expected period? That’s because most people would have no idea when exactly they ovulated. If you are one of the women who has been tracking ovulation and knows when you ovulated, it’s still worth waiting until at least 12 days past ovulation to test for a few reasons.
Implantation dates can make a difference to when a test will show as positive
If you have sex around the time of ovulation and an egg is fertilised, it still needs to implant into the uterine wall before a pregnancy can stick. While implantation can occur as early as 6 days past ovulation, in the majority of cases implantation doesn’t take place until 8-10 days past ovulation. As the hCG hormone won’t begin to be produced until after implantation and will take a number of days to get to a level where it will register on a home pregnancy test, it is easy to see how taking a test too early will throw up a false negative.
The Negative Impact Of Testing Too Early
There is an obvious financial implication for testing too early. Pregnancy tests can be really pricy and testing too early will almost certainly show a negative result.
Beyond the financial impact, we have to consider the emotional impact of testing too early. Not only can it be super depressing to see negative after negative result when all you want is a baby, but it can cause additional upset too.
You can suffer from “line eyes” squinting for hours at faint lines that may or may not exist (true story, I have done this over and over).
Testing early with super-sensitive tests can also show a faint positive for a pregnancy that will never stick and will sadly end as a “chemical pregnancy”. These chemical pregnancies would rarely be far long enough to show up any symptoms and would simply present as a normal period. As hard as having a visit from “Aunt Flo” or AF can be (yes, “Aunt Flo” is what your period is called if you venture into Trying To Conceive forums), it is not nearly as traumatic as seeing a positive test they grieving a lost baby.
Hard as it may be to hear
Waiting until 14 day past ovulation or the first day of a missed period to take a pregnancy test really is for the best for your wallet, your head and your heart too. The Two Week Wait can be agony, trust me, I know, but if you can manage to wait it out, it will mean that the results of the test are far more likely to be accurate, meaning your “Big Fat Positive” or “BFP” as it’s known, it’s far more likely to mean that you really are pregnant.
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