Understanding home pregnancy tests

Many women know that they are pregnant from the fact that they have missed a period along with some tell-tale side effects such as nausea, breast tenderness, bloating and lower abdominal cramping but how is a pregnancy confirmed? Well the most reliable way to confirm a pregnancy is to carry out a urine or blood test. Both tests are looking for the hormone HCG (Human chorionic Gonadotropin) which is produced by the placenta.

Urine Tests

These usually come in 3 different types; Strips, cassettes and midstream tests. All of these work to detect HCG in the urine in the same manner – via a little strip within the test that turns blue or pink if enough of the HCG hormone is detected.

Pregnancy test sensitivity

Now many women get confused with pregnancy tests especially when they are very early in pregnancy and take a few tests with some reading positive and some reading negative. Imagine taking a pregnancy test one morning and its positive, you then take another in the evening to confirm and its negative. How bizarre is that? But it happens more frequently than women think. This can be because of the  sensitivity of the test. Before buying a pregnancy test, check its sensitivity. Pregnancy tests usually detect either 10miu or 25 miu of the pregnancy hormone HCG in urine. The 25 miu tests can usually be performed a day after a missed period and the 10miu ones can sometimes read positive a few days before a period is even missed.

HCG

The body produces the hormone HCG from the placenta that starts to form once the fertislised embryo has attached to the wall of the uterus. The implantation process takes around 6-12 days after conception, this is why a pregnancy test will not be positive until a while after intercourse has taken place.

False positive tests

It is unlikely that a urine test will ever be a false positive. HCG levels in humans are usually below 8miu unless there is another medical issue apart from pregnancy such as certain cancers. We recently had a question from one of our members who also submitted pictures of her tests:

So I took a pregnancy test yesterday and it was negative, so I took a digital one this morning and it said Yes+ so I went and got another one about an hour later to make sure and they both said negative. Do you think this means I’m pregnant or possibly a false positive?

positive pregnancy test

Positive pregnancy test

False negative pregnancy test

False negative pregnancy test

As you can see these are digital tests but different brands. The first says yes + meaning pregnant but the second one reads negative. As its unlikely to be a false positive test, the advice is to wait 48 hours and try again using the first early morning urine sample.

The non digital tests require you to read the result yourself. The general rule is, if you can see a line, no matter how feint, then its positive. Some tests are cheaper than others but they all work in the same way!

Taking more than one pregnancy test

You can see by the image of all these tests below taken within a few days of each other from 12 days post ovulation (12dpo) to 15 dpo, that the test line gets darker as time goes on. This is because the levels of HCG double every 48 hours making the concentration of the hormone higher in the urine.

faint Positive pregnancy test 13 dpo to 15dpo

Image source: Netmums

 

It is therefore possible to have a false negative test. If a test is taken after a missed period and is negative but a pregnancy is still suspected, the test should be performed again after 48 hours. The conception date varies from woman to woman so if a woman hasn’t ovulated and conceived until late in her cycle for example day 25/26, she wont have a positive pregnancy test until around a week or more after a missed period.

HCG levels after conception

HCG levels after conception

HCG levels

You can see by the chart that even at 28 days from the last period which is the time many women take a home pregnancy test, some women’s HCG levels are not even detectable via a standard urine test.

 

 

If a home pregnancy test is still reading negative 2 weeks after a missed period then it is worthwhile visiting the doctor to request a blood test which may prove more accurate.