A general overview of DIY pregnancy tests
Home pregnancy tests are supposed to detect the pregnancy hormone hCG (human chorionic gonadotropin) in the hopeful mum's pee. hCG is made in the placenta after the fertilised egg is implanted.
Essential Baby explains the two different types of tests available: qualitative and quantitive.
"A qualitative test picks up on units of hCG above 25 (measured in mIU per millitre). It can only determine if hCG is in your urine – it can't give you a reading of the level of hCG. Pregnancy tests that show two lines, a happy face or the words 'yes' or 'pregnant' all fall into this category," the Essential Baby article explains.
"A quantitative pregnancy test, however, will not only confirm that you're pregnant, but ... there are also some digital home pregnancy tests that can give you an estimate of how many weeks you are (these will display either 1, 2 or 3+ weeks in the window)," says Essential Baby.
How accurate are home pregnancy tests? If they are used strictly according to the instructions on the pack - especially when it comes to when is the best time to take a pregnancy test - the results should be 97% accurate, but things can go wrong.
Here are some reasons for a false positive pregnancy test or why the result you get may not be right:
Is it possible to get a false positive pregnancy test?
Yes, it is rare, but it can happen that you can you get a false positive pregnancy test. Better Health Victoria lists several reasons why a pregnancy test may record a "pregnant" result, even if you are not actually pregnant.
1. Contaminated equipment
Detergent on the cup you collect the pee in may cause a false-positive. It is probably best to test the stick direct in the urine stream, or make sure any equipment you use is washed, dried and wiped clean.
2. Defective tests
Check the use by date on your test before using. Also they may not work if the tests have been mishandled - leaving them open in bathrooms means they may have been exposed to heat or moisture.
3. Other medical issues
If you are unwell, you may have blood in your urine (such as with cystitis) or protein (such as with kidney damage) that can cause an incorrect reading. Also ovarian cysts and menopause can affect results. And there are rare cases of it occurring in phantom pregnancies.
The UK NHS website lists the following medications which may cause DIY tests to show a positive result without pregnancy. It advises reading the information leaflets or checking with your pharmacist to see if drugs you are taking can affect your results.
- allergy medication such as promethazine
- drugs to treat Parkinson's disease
- diazepam and other anti-anxiety medications
- clozapine and other anti-psychotics
- epilepsy medications
- infertility medications
If you're taking any regular medications, it might be best to check if they can affect pregnancy test results.
5. Previous or non-viable pregnancy
If you have had a baby not long ago or a recent miscarriage, the pregnancy test may still pick up hCG for a few weeks afterwards, causing a false positive result.
Similarly, if you take the test early, it may pick up a pregnancy that would not have progressed. This is called chemical pregnancy and is very common: if you hadn't taken the test, you would never have known you were pregnant.
There is also the possibility of an ectopic pregnancy, where the egg implants somewhere other than the uterus. This can be a serious medical condition and the pregnancy can not be carried to term.
6. Undiagnosed medical conditions
The presence of hCG could be down to the possibility of an ovarian tumour or a molar pregnancy, where there is no baby but a growth instead. That's why checking pregnancies out with a doctor's visit is always a good idea.
And, as always, the test is only as accurate as the way it is used. One more reason for an incorrect positive result:
7. Not following the instructions
Sometimes if you leave a test for too long, it can seem like a second line has appeared, due to evaporation. If you are not sure if this is what has occurred, take a second test and follow the viewing instructions carefully.
If you get a positive pregnancy test, and you have followed the manufacturer's instructions carefully, it is always best to follow up with a medical professional to confirm.
Is it possible to get a false negative pregnancy test?
Yes. The Mayo Clinic website says it is far more likely to get a false negative result than a false positive result. Here are the most common causes:
1. Taking the test too early
Although the temptation to find out one way or the other is strong, if you take the test before the recommended time, the levels of hCG may not have built up enough to be detected. So you might get a negative result but actually be pregnant, just not pregnant enough to show up. Better Health Victoria recommends waiting till your period is at least a week late, though 2 weeks past period should confirm.
2. Not waiting long enough before checking the result
The tests take a while to work. Better Health Victoria recommends using a timer or a watch with seconds to make sure you wait exactly according to the instructions. (See above how waiting too long can give a false positive.)
3. Not following the instructions
Reading the info that comes with your test was never so important. Essential Baby warns of what can go wrong if you don't: "If any of the steps are carried out incorrectly, if urine splashes on the results window, or the test isn't laid on a flat surface, the results can be affected."
4. Watered down wee
If you have drunk a lot of water, it can affect the levels of hCG in your urine. That's why it is recommended to take the test with your first wee of the morning, which is usually the most concentrated.
5. Waiting too long
As described by Ava Woman, there is also the possibility of the hook effect - where the hCG levels are too high to detect - this can occur after five weeks meaning sometimes you can have a positive test then negative.
If you feel pregnant but had a negative test - maybe you have the symptoms of pregnancy like nausea, tiredness ... no period - try taking a follow up test or head to see a medical professional.
What is the best way to get a 100% accurate pregnancy test?
The best way to get results that are as accurate as can be is to make sure you follow the instructions that come with your test as carefully as possible.
And for more accurate results, the Mayo Clinic reckons it is better to hold off taking the test till at least the first day your period is due, if not a week later.
"Why wait? ... During early pregnancy, the HCG concentration increases rapidly — doubling every two to three days. The earlier you take the home pregnancy test, the harder it might be for the test to detect HCG," says the Clinic.
"Keep in mind that the exact timing of ovulation might vary among women or even from month to month, and the fertilized egg can implant in the uterus at different times. This can affect when HCG production begins and becomes detectable."
So the more pregnant you are, the more accurate a urine test will be.
Doctors usually recommend a blood test to confirm - you can have one from 11-14 days after ovulation and they are usually 99% accurate.
Or you can have an ultrasound - where you can hear the baby's heartbeat - which can confirm the pregnancy is viable from about six-and-a-half weeks.
Seek help if you continue to miss periods or have a late period
If you are trying to get pregnant or are even just concerned by your lack of cycle, it's a good idea to get checked out by a doctor or medical professional.
"Many factors can lead to missed menstrual periods (amenorrhea), including thyroid disorders, low body weight, problems with your ovaries, excessive exercise and stress. If you're not pregnant, your health care provider can help you get your menstrual cycle back on track," says the Mayo Clinic site.