How Do Pregnancy Tests Work?
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How Do Pregnancy Tests Work?

I’ve always thought it amazing how with
just a piece of paper, pee and five minutes of your time you can find out whether you’re
pregnant or not. However, as simple as pregnancy tests are
to use, they are actually a feat of biochemical engineering, and today I’m going to tell
you exactly how they work, and how they can not only inform us about pregnancy, but they
can also help us with the diagnosis of prenatal conditions and certain types of cancer! [Intro by Cristina de Manuel & Caro Waro]
In Ancient Egypt, women used to pee on barley and wheat plants to find out if they were
pregnant, and a few days later, if the plants had grown, it was a sign that they were pregnant,
and the plant that had grown the most would determine the sex of the future child. Now, whilst this may sound like a p1$$-take,
the Egyptians were actually on the right track when using urine as a means for detecting
pregnancy, and even more surprisingly, a modern study showed that the plant growth was accurate
in detecting pregnancy on 70% of the occasions. [Not so much so for detecting the sex of the
future child mind you!] Now, the reason for this is that a hormone
produced early on during pregnancy is capable of stimulating growth in plants – and very
conveniently, is excreted through our urine. This hormone is the human chorionic gonadotropin
hormone – or hCG for short. Our bodies produce very low levels of hCG,
but after the implantation of a fertilised ovum in the placenta, the concentrations of
hCG in a pregnant person’s body dramatically increases, and this hormone is crucial to
signal to the body that it’s time to kickstart pregnancy. hCG is what is known as a heterodimeric glycoprotein. The glycoprotein bit means that it’s a protein
with a carbohydrate group attached, and the heterodimeric part means that it is formed
of two different subunits, alpha and beta. In hCG, the alpha subunit is common to all
glycoproteins of the same family, including follicle-stimulating hormone, luteinising
hormone and thyroid stimulating hormone – but the beta subunit is unique to hCG – and
that is precisely what the pregnancy test strips are designed to detect. The way pregnancy tests do it is by using
a so-called ELISA test, which stands for enzyme-linked immunosorbent assay. Its job is to detect a particular substance
within a particular liquid, and in this case, the substance is the unique beta unit from
hCG, and the liquid is urine, and it’s a sandwich because said liquid must pass through
three zones lined with antibodies, with the important test happening in the middle. Antibodies are very special biological chemicals,
made up of a constant zone, and a specific zone, which is designed to bind to specific
compounds. The first zone is the reaction zone. It is lined with antibodies that can bind
with hCG. As the urine passes through, these are swept
up, and if hCG is present, the antibodies will bind to it. The second test zone is lined with antibodies
that detect antibodies that are bound to hCG. These are picked up and cause dye release,
which is the positive pregnancy test line that can be both exciting and terrifying for
many! The final test zone is the control zone, this
one simply detects the constant zone of the passing antibodies to ensure the test has
run correctly, and as they detect the antibodies from the reaction zone passing through, they
release dye. This line basically tells us that the test
has run correctly. Now, so far I’ve spoken about urine tests,
which are your traditional over the counter tests. However there are also serum tests which take
blood, and they can detect even lower concentrations of hCG, and unlike urine tests which are generally
qualitative tests, serum tests can quantify the levels of hCG. There are various reasons that you may want
to do that. For one, it can actually tell you exactly
when conception happened, but they are informative in other contexts as well. For instance, some studies have found links
between persistently high levels of hCG in the first trimester and Down syndrome. hCG tests are also helpful for detecting and
monitoring ectopic pregnancies and miscarriages and high hCG levels can also be a sign of
certain types of tumours and cancer, such as pituitary gland cancers or testicular cancers,
and measuring hormone levels is a very safe and non-invasive way to monitor them. Finally, hCG tests can also be used in anti-doping
contexts, as some male athletes may use hCG to stimulate testosterone production due to
its similarity to luteinising hormone. So without a doubt – how much a simple hormone
can tell us about the inner workings of our bodies can be both relieving and mind-boggling
– and it’s all thanks to biochemistry! And hey! In the time it’s taken me to tell you all
of this – my test is ready to interpret! Negative! I really hope you enjoyed today’s episode
– this was actually my entry to the Biochemical Society’s Science Communication Competition
back in April aaaand it came in third place – so I hope you enjoyed it. if you want to see the first and second entries
and I think a few written entries as well, I will link those down below – and as always,
thank you so much for watching me and I’ll see you in the next one! Bye! [Music: Thastor & CryoSleepKitten]
[Art & Animation: Caro Waro & Cristina de Manuel]
[Hosting, Scripting, Editing: Inés Dawson] [Translated by {your name} into {language}]


  • Draw Curiosity

    I'm very honoured that the Biochemical Society chose this video as one of their winners for their Science Communication Competition. 🙂
    By the way, I've also been starting to use SnapChat Stories to film snippets of my daily life as a PhD student – if this interests you, feel free to follow me on SnapChat under the username squeakcode (SnapChat is a mobile app only I'm afraid)

  • flagpoleeip

    is it really called a placenta when an egg implants? at that stage is it not just endometrium? or does it immediately become placenta by definition?

    great video. it's not something I'd ever thought about.

  • skizm

    haha, geez. when you started talking about "my test" it made me think this video was a very elaborate way to announce you were pregnant 😀

  • Maxx B

    Great video. I enjoyed the bleeped out pun. I'm so glad they don't have to inject urine into rabbits any more. That's why it's a good idea to keep funding science and research.

  • ossiebird0

    Pregnancy tests can give a false negative up to 6 weeks into pregnancy as my partner and I found out with our first child. Tests are very unlikely to give a false positive.

  • Karl Fernandez-Cao

    I rolled my eyes at the title of the video thinking I knew it all, but then you take it 10 steps better and really educated the hell out of me. Now I want to do more research on hCG.

  • belg4mit

    I think the judge's got the order reversed. This was better produced and more entertaining than the other video available. Congratulations nonetheless.

  • Mike W

    I managed to break my hospital's HCG and quantitative HCG tests last week. Now that you've told me more about how they work, I'm especially glad I got them fixed really quick. Thanks Ines!

  • HistoricaHungarica

    Are there any hormones that can not be tested with an ELISA test? (sure, the ones that urine, saliva or sweat does not contain… but i'm more curious about the ones that aforementioned liquids DO contain but the tests are unable to measure accurately)

  • Gillian Walker

    I'm going to save this and watch it again. There was soo much to learn, I'm sure I didn't get it all the first time.

  • 2300Kenzie

    Having performed thousands of hCG tests in clinical labs, I appreciate your clear explanation. To me one of the worst outcomes is a false negative test. This can occur due to poor technique or contamination.  As long as the instructions are carefully followed, it is very accurate.  Excellent video.

  • aron craig

    Definitely like your video better than the 1st place. His was a good concept, the style was nice, and very timely but it certainly could have use a bit more polish (particularly in the audio).

  • Pseudonymium

    Very interesting, I never knew that a pregnancy test it just an ELISA assay of sorts. I wonder what kind of antibodies they are (species, isotype etc.) and if they are lyophilised on the paper then redissolve in the urine. I'll have to do some googling.

  • Professor Politics

    I wonder if those tests the ancient Egyptians used were actually more accurate in the context of other clues (missed period and all that)–kinda a Bayesian inference idea. Really interesting video!

  • No One

    raios me es muy dificil concentrarme con esos ojitos y esa sonrisa tan linda… excelente video muy informativo… gracias por compartir sigue asi exitos …Saludos desde Perú …somos muy pocos los suscriptores… compartan los videos gente

  • Kevin Octacok

    I know this is a bit off topic, but I could listen to literally anything said with that British accent of yours! Why is it that us Americans generally really enjoy British and Australian accents in general?

  • sami huttunen

    I found your channel thanks to Pharyngula (the awesome blog of PZ Myers) and this was the first – but certainly not the last – video I watched. Amazing stuff, entertainingly made. Thank you and warm greetings from cold Finland!

    P.S. Your video gave me a strange idea for a "song". If you ever happen to need evil/dark/weird soundtrack stuff, I'd be honored to contribute some. I'm not a pro but my electronic flatulence has been used in some short movies and elsewhere. Not yet in science videos and I would love to be involved.

  • Lisa Parrott

    I don't think it's called a sandwich because there's 3 tests… Doesn't it have to do with the hCG being caught between two antibodies?

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