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How do pregnancy tests work_
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Pregnancy has been in the news over the past month, with a royal baby back in October and the usual smattering of celebrity pregnancies. The inspiration for this post, however, originated a little closer to home; let’s just say that, in around 6 months’ time, my wife and I are going to be entering a fun new world of sleep deprivation! Pregnancy brings up a whole host of science and chemistry questions, the first of which is: how do pregnancy tests work?

For some women, the first signs of pregnancy come before the use of a pregnancy test. This can be in the form of tiredness, a missed period, or even nausea and vomiting. Home pregnancy tests can confirm the suspicions these may generate, with a claimed 99% accuracy. The most reliable results come a week after a missed period. However, some pregnancy tests can confirm that you’re pregnant earlier than this, as early as 8 days after conception.
Pregnancy tests work by detecting the presence of the hormone, human chorionic gonadotropin (hCG). hCG is a hormone produced by cells in the placenta. Its production starts from the point at which the developing embryo attaches to the uterus, 6-12 days after conception.
hCG’s primary role is to support the function of the corpus luteum, a temporary structure in the ovaries. The corpus luteum is essential during pregnancy. It produces the important hormones, estrogen and progesterone. Progesterone relaxes the muscular wall of the uterus and prepares its tissue lining so that the embryo can implant. Estrogen is key for the development of the foetus, stimulating organ growth and thickening the lining of the uterus.
hCG concentration increases from conception, and peaks between 8-11 weeks after conception. For the first few days after conception, its levels can be too low to be detected by pregnancy tests. After implantation its levels double approximately every 48 hours, making it much easier to detect.
hCG is eliminated from the mother’s body through urine, and this is what pregnancy tests use to detect it. Urine is applied to a sample pad at the end of the test stick and is drawn up the strip. The first area it reaches is the reaction zone. If hCG is present in the urine, here it binds to proteins called antibodies. These antibodies have an enzyme attached to them which can participate in reactions further along the test strip.
The test zone contains different antibodies which are attached to the strip and unable to move. These antibodies also bind to hCG, creating something of a hCG sandwich between the two different antibodies. The enzyme on the mobile antibodies triggers a colour change in dye molecules on the test strip. This line only appears if the urine contains hCG – and hence, if the woman is pregnant. If there’s no hCG, the mobile antibodies just sail on past.
Whether the woman is pregnant or not, another line appears on the test in what’s known as the control zone. Here, more immobile antibodies bind to the excess mobile antibodies that didn’t pick up hCG and continued travelling up the strip. Again, this triggers a colour change. This proves that the test is functioning correctly – whether a woman is pregnant or not, the control line should show.
Digital pregnancy tests have become more common in recent years. Though these may look different from the outside, they work in exactly the same way. They simply use a sensor to detect the colour changes and determine whether to display ‘pregnant’ or ‘not pregnant’ on their screens.
Though home pregnancy tests seem commonplace now, they were only developed in the 1970s. Prior to this, tests had to be carried out in laboratories. These laboratory tests were developed less than 100 years ago and were initially somewhat weirder than today’s methods. The first test which could accurately detect pregnancy involved injecting a woman’s urine into several immature female mice. If the woman was pregnant, the ovaries of the mice would grow and produce eggs. Later developments of this test substituted frogs for the mice, in what became the first widely used pregnancy test. Waiting for results took up to a week – a far cry from the mere minutes home pregnancy tests operate in!
Even these first laboratory tests weren’t as odd as some of the methods used to predict pregnancy prior to their development. In the Middle Ages, uromancers (given the fantastic nickname of ‘piss prophets’) tried to divine whether a woman was pregnant. They did this via a number of methods, including whether a woman’s urine rusted a nail or changed the colour of leaves. It’s likely none of these methods were particularly effective.
There are, however, some ancient techniques which had a degree of accuracy, even if at the time people didn’t know why. Ancient Egyptian women reportedly urinated in bags of barley and wheat seeds. If either of the seeds sprouted quickly, this indicated pregnancy. This had a surprising success rate of 70-85% – scientists believe this is because estrogen in the urine stimulated the seed growth.
Happily, women today don’t have to urinate in bags of seeds or have their urine injected into mice to determine if they’re pregnant. Today’s home pregnancy tests are so accurate that doctors often won’t need to follow up a positive result with further confirmatory tests.


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