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Pee is for pregnant: The history and science of the urine-based pregnancy test

Pee is for pregnant: The history and science of the urine-based pregnancy test

8 min read

Let me look at your pee, and I’ll tell you your future.

For over 3000 years, women wondering if they’ll be mothers have heard some variation of this phrase. In the first known pregnancy tests, ancient Egyptian women urinated on barley or wheat seeds: quickly sprouting seeds indicated pregnancy. While this may sound like pseudoscience, several modern studies have shown that it works pretty well, correctly identifying 70-85% of pregnancies.

The ancient Egyptians were certainly onto something, as modern pregnancy tests work in much the same way: they detect a hormone in urine. But it’s still taken thousands of years of scientific progress to get us from peeing on seeds--and waiting a week to see if they sprout--to the fast, convenient pregnancy tests found in all pharmacies today.

“Piss Prophets”: Early history of the pregnancy test

Although it’s not intuitive that analysis of urine would be the key to pregnancy detection, the majority of historical pregnancy tests have, for unknown reasons, focused on just that. In Europe, starting in the Middle Ages, “Piss Prophets” claimed to be able to predict pregnancy with a variety of bizarre urine tests. They believed that pregnant women’s urine would rust a nail, change the color of a leaf, or be home to tiny, living creatures. Based on what we know today, it’s unlikely that any of these European tests were actually able to correctly detect pregnancy.

However, the first of the modern tests that could accurately detect pregnancy was no less bizarre. This test was developed in 1927 by German scientists, Aschheim and Zondek. They found that injecting a pregnant woman’s urine into sexually immature female mice would cause the mouse’s ovaries to grow and produce eggs.

What’s so Special about Pregnant Women’s Urine?

If you’re like me, you’re now left with a burning question that you probably weren’t expecting to ask today: why does a pregnant lady’s pee make a juvenile mouse ovulate?

The short answer is that the urine of pregnant women has a unique blend of hormones. A woman (actually, anyone with a uterus, though throughout the article I’ll refer to women) becomes pregnant when an egg released from her ovary is fertilized by sperm and the fertilized egg implants in the uterus. This fertilized egg then starts growing into an embryo and releases a hormone called hCG (a.k.a. human chorionic gonadotropin), which builds up in a woman’s body during early pregnancy. Like all hormones, hCG sends a signal. hCG’s particular job is to tell the uterus to stop the woman’s regular menstrual cycle and prepare itself for pregnancy.

Since hormones are excreted into the urine after they’re done sending their signals, the urine of pregnant women has hCG in it, whereas the urine of anyone who is not pregnant does not. hCG, it turns out, makes mice ovulate, allowing Aschheim and Zondek’s test to detect pregnancy. Since their test, all modern pregnancy tests have focused on detecting hCG.

But why does hCG make young mice ovulate? hCG is very similar to another hormone called luteinizing hormone. In humans and in mice, luteinizing hormone stimulates the ovaries to release an egg every month of the menstrual cycle, and hCG can mimic that signal. In fact, because it can mimic the ovulation signal, hCG is used as a fertility treatment in women trying to conceive. Aschheim and Zondek’s test worked because the level of hCG in the urine of pregnant women is much higher than that of luteinizing hormone in non-pregnant, ovulating women:

Mice, Rabbits, and Frogs: Animal-based pregnancy tests

Although Aschheim and Zondek had developed an accurate test, it wasn’t exactly simple, especially compared to the drug store tests available today. They had to inject five mice per woman and wait about a week before getting the result. And even then, they could only detect the high levels of hCG that women have starting around 2 weeks after a missed period. In addition, the use of so many animals made it so that the test was expensive and relegated it to a few labs that received shipments of urine through the mail.

Today, woman can know that they are pregnant as soon as 10 days after conception—a few days before a missed period--using tests they can buy for less than $10 in any drug store. But up through the 1930s, women had to wait until at least a month after conception, visit a doctor, mail her urine to a lab, and then wait at least another week to get the result of the test. Unsurprisingly, pregnancy testing in this era was not routine, used only by those who were wealthy or needed to know if they were pregnant for medical reasons.

The juvenile mouse test was slightly improved in 1931 by the American doctor, Maurice Freidman, who swapped juvenile mice for adult rabbits, which were easier to inject.

However, a test using frogs, developed the British scientist Lancelot Hogben, marked the pinnacle of these animal tests. Frogs lay eggs, so they don’t need to be killed or dissected to assess ovulation and could therefore be reused, lowering test costs. This test also gave results faster: within twelve hours. The frog test increased the availability of pregnancy testing, but it still required shipping urine to a select number of frog labs.

Tens of thousands of frogs were injected with urine throughout the 1940s-1960s, but pregnancy testing in this era was still not the norm. Most labs would only test urine sent by a doctor, meaning that women had to rely on their doctors to get tested. And many doctors—and health insurance companies--would only grant a woman a test if she had some urgent medical reason that she needed to know she was pregnant. Most women instead relied on morning sickness and sore breasts as early clues to their pregnancy, not visiting a doctor to confirm until months after conception.

Blood and Test Tubes: The pee-on-a-stick precursors

This all changed in the 1960s when scientists began to develop pregnancy tests that didn’t require the use of live animals and could therefore be done in doctors’ offices. At first, these tests had similar sensitivity for hCG, but by the early 1970s, Drs. Vaitukaitis, Braunstein, and Ross had developed a test that could detect pregnancy just 2-3 weeks after conception—on the first day of a missed period. These new tests used antibodies.

Antibodies are molecules that recognize and stick to other molecules. In their usual job, they’re sticking to the molecules of bacteria or viruses to alert the immune system to these invaders. In pregnancy tests, however, scientists engineered antibodies that recognized and stuck to hCG. Armed with these hCG antibodies, the urine of pregnant women, and, strangely enough, sheep’s blood, we had a new pregnancy test.

Scientists attached hCG to the outside of the sheep’s blood cells, decorating the blood cells with hCG. They then mixed these blood cells with the other test components: hCG antibodies and urine.

Based on the way that antibodies stick to hCG and to each other, the hCG-decorated blood cells would clump if they were mixed with the urine of a woman who was not pregnant. On the other hand, if the blood cells and hCG antibody were mixed with the urine of a woman who was pregnant, the blood cells would not clump. So by looking at the clumpiness of the blood cells, doctors could tell if a woman was pregnant.

These and other, similar antibody-based tests could give doctors results in a few minutes to hours and finally made pregnancy testing mainstream. At first, women still had to visit their doctor to get a pregnancy test.

But that, too, changed in the 1970s (1971 in Canada, 1977 in the US) when the first at-home pregnancy test hit the market, using this same blood-and-antibody technology. This test looked like a kid’s chemistry kit, including test tubes, droppers, dried capsules of sheep’s blood cells, and hCG antibody serum. Success in completing the test required carefully following a 10-step process all while keeping the test tubes in a place free from vibrations for two hours. But if used correctly, it was 97% accurate for a positive result and 80% accurate for a negative one. And for the first time, a woman could confirm that she was pregnant without ever contacting a doctor.

Pee on a Stick: The birth of the modern pregnancy test

Since the 1970s, at-home pregnancy tests have become the norm, with about 20 million sold per year in the US alone. Current stick tests, which were developed in 1988, are much easier to use, don’t involve sheep’s blood or test tubes, and are still extremely accurate: about 99% for a positive result if used correctly. But they’re still doing what pregnancy tests have been doing for thousands of years: detecting a hormone in women’s urine. And modern tests are really specific for hCG: the rare false positives usually occur because the test detects hCG in the urine coming from something other than pregnancy: this could be from hCG-based fertility treatments or due to certain medical conditions. Such false positives can be identified by repeating a urine test later in the pregnancy or by taking blood tests for hCG available at doctors’ offices, which can provide more information about how much hCG is in the body.

Like the 1970s tests, modern pregnancy tests still use hCG antibodies, but instead of interacting with hCG-decorated blood cells, the antibodies interact with dye-activating molecules. These dye-activating molecules are what causes the blue line, or other symbol, to appear on positive pregnancy tests.

Women today usually find out that they’re pregnant just after (and sometimes before) missing a period in the comfort of their own bathroom without the involvement of a doctor. But as recently as the 1950s, women could not definitively confirm pregnancy until they started to noticeably gain weight or feel the fetus move—several months into the pregnancy. And until the 1970s, women often relied on doctors to tell them that they were pregnant.

The development of the pregnancy test was not just a feat of scientific advance, but also allowed for a cultural revolution. The at-home pregnancy test became available at a time when women were fighting back against the male-dominated medical establishment, trying to learn more about their health and their bodies. And it granted women more information about and control over their bodies. Women could use this information as they liked, without pressure from a doctor: they could decide for themselves to get a safe, early abortion or start prenatal care.

And this cultural revolution continues beyond pregnancy testing. Starting in the 1960s, technologies for detecting hormones in urine and blood aided in the development of hormonal contraceptives, which allowed women to choose if and when to get pregnant. Today, continued improvements in hormone testing allow women to track their fertility over the course of their lives, making it possible to plan to have children at a time that’s both personally and biologically feasible. And when women do choose to become pregnant, our understanding of hormones comes in handy once again in the form of ovulation kits that allow women to track fertility throughout their cycle, facilitating the process of conceiving.

As a woman who is grateful for these continued cultural advances, I want to say thanks to all the doctors and researchers who have uncovered the mysteries of our hormones -- and especially to the first person who thought, “I bet I can tell if she’s pregnant by looking at her pee.”

Kelsey Tyssowski is a graduate student in the Department of Genetics at Harvard Medical School. You can find her on Twitter @kelseytyss. An earlier version of this piece was written for Science in the News.

For more information:

  1. A Thin Blue Line: a pregnancy test history put together by the NIH
  2. An academic article about the science and culture of animal-based pregnancy tests
  3. An excellent, but hard-to-access, academic review article about the history of the pregnancy test, and a less-descriptive, but similar, open-access article
  4. An article in The Atlantic that describes the history of the pregnancy test from a more historical perspective
  5. A video explaining the science behind modern pregnancy tests

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Kelsey Tyssowski

Kelsey Tyssowski is a graduate student in the Department of Genetics at Harvard Medical School. She is also a former editor-in-chief of the SITN Blog. You can find her on Twitter: @kelseytyss

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