This test shows what you’re like inside…

Just how do pregnancy tests know you’re pregnant?

We’ve all seen an advert for a pregnancy test, I’m sure. They usually involve a couple of young women or a couple grinning over a stick which has some lines on it. They’re a quick and easy way to find out if you’re pregnant (and even sometimes roughly how far into pregnancy you are). Not many people actually stop to consider how they work though. So look no further, for in this post I shall tell you all about the science behind pregnancy tests! Yay!


Pregnancy tests measure the levels of human chorionic gonadotropin (hCG) in the body. This is a hormone that lets the body know that conception has occurred. If a female is not pregnant, then at this stage the corpus luteum breaks down, and leads to menstruation. The corpus luteum is basically the remains of a follicle which contained the egg which has been released. If pregnancy has occurred, hCG stimulates the corpus luteum to continue secreting progesterone (another hormone), which maintains the lining of the uterus ready for implantation of the fertilised egg and subsequent growth of the embryo.


hCG is detectable in the mother around 8 days after ovulation, and its levels rise rapidly (almost doubling) every couple of days after until around 9 weeks into the pregnancy, when its levels flatten out and remain stable. It’s easy to measure its levels using the ‘dipstick’ style pregnancy tests which are easily available over the counter for people to use. It’s also why they can predict a timescale as to how long somebody has been pregnant. Relatively low levels mean only a couple of weeks, whereas higher levels could mean someone is already a month into their pregnancy.


The tests measure the hormone levels by containing molecules which bind to any hCG which may be in the urine sample. These then bind to another molecule, and if the result is positive, the colour change occurs which lets a person know they’re pregnant. Most tests also contain a control strip, which either backs up the result or lets somebody know that the test may be faulty so they should take a new one.


However, it is always advisable to go and see a professional to confirm these things, as hCG can have other uses in pregnancy too. Its levels can be monitored in a clinical setting to check on the health of the foetus. Very low levels of hCG can indicate that a pregnancy is ectopic (that is to say, the egg has not implanted in the uterus but elsewhere in the female reproductive system, such as the fallopian tubes). This can lead to complications if not picked up, and hCG is a really nifty indicator that all is perhaps not as it should be in the pregnancy. Conversely, abnormally high levels of hCG indicate that there are perhaps multiple implantations, and that someone could be pregnant with twins/triplets etc.


Of course, getting pregnant isn’t always easy. This could be due to a variety of reasons. However, a quick way that people are now trying to improve their chances of conceiving is by using something similar to pregnancy tests – ovulation tests. These can predict when an egg is going to be released, making it more likely that fertilisation can occur.


hCG is not measured using these tests. Instead, LH is. LH (Luteinising Hormone) is probably one of the ones that you remember (alongside FSH) from when you had to learn the menstrual cycle in school. It stimulates the release of an egg from the ovaries. In fact, the so-called ‘LH surge’ is a huge increase in levels of the hormone from around 36 hours before ovulation occurs. Therefore, by testing for LH using another over the counter test, people can predict when they’re most likely to have the best chances of conceiving. Clever, huh?


So there you have a brief summary of how over the counter ovulation and pregnancy tests work. Why not explain how they work next time someone tells you they’re pregnant? But remember to congratulate them too, that’s usually what they’re expecting.


If you’re interested in a bit more information and history about pregnancy tests, there’s a great TED-Ed talk on them here.

If you want to chat to me about anything, really (or request future blog posts) my twitter is here.



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