The Basics of hCG
Home pregnancy tests are made possible by the “pregnancy hormone”: human chorionic gonadotropin (hCG). A woman’s body begins to produce hCG after a fertilized egg successfully implants into the uterine wall. Numbers rise quickly, doubling about every two to three days. Home pregnancy tests work by detecting hCG in urine, and different brands can detect different levels – for most women, the lower the hCG levels the better, because this means they can learn about a pregnancy sooner. More sensitive tests might pick up hCG at just 10-15 mIU/ml (milli-international units per milliliter), while less sensitive tests might not pick it up until levels reach 50 or even 100 mIU/ml.
Urine pregnancy tests are qualitative, meaning they simply ascertain whether the body has produced any hCG; they answer a yes-or-no question. If hCG is present, at any level equal to or higher than the threshold the test is capable of detecting, the test turns positive. (A faint or barely-there line is thus a definitive positive, because it indicates the test did indeed detect hCG. I learned this in my first pregnancy.) In comparison, blood tests are quantitative; they measure the body’s overall levels of hCG, giving you a number rather than a yes/no. They have the potential to give some indication of gestational age, but hCG levels vary so widely that any given number can only be nominally telling.
More important than any static hCG measurement (a one-time test on a single day) are the results from serial hCG testing (tests performed daily for two or three days in a row), since hCG should be increasing quickly in early pregnancy. In the case of any cause for concern (such as bleeding), serial hCG testing is a common approach. Numbers should climb.
The classic escalating hCG patterns were first documented in the late 1930s. Most studies offer similar values and ranges by week, with peak levels around weeks eight or nine, although there is a tremendously wide range of “normal.” Beyond the first trimester, hCG levels plateau and then fall off. (There is evidence that women carrying multiples tend to have higher hCG levels, on average, compared to women carrying singletons, but this is just a correlation – any one hCG measurement or set of hCG measurements cannot reveal an embryo count. The only way to do that is with an ultrasound.) Because there is no exact model of hCG growth, a Google search yields many examples. Here is one template from the American Pregnancy Association.
If you’re like me and want even more specifics, you might like to check out the online “Betabase” at http://www.betabase.info. It’s an interactive website where visitors can enter their hCG measurements and corresponding gestational age; the outcome is a wealth of data about average, high, and low hCG levels at any given point in early pregnancy. It’s not scientific, strictly speaking, but it contains data for almost 120,000 pregnancies. Plus it’s fun.
Home Pregnancy Tests
It is hCG that has afforded women the luxury – or the burden, depending on your point of view – of using early home pregnancy tests. Before the 1970s, when home testing kits were introduced in the U.S., pregnancy was confirmed either through a period of waiting or, after the 1920s, by laboratory analysis. The first test that could identify hCG in humans was developed in Germany; it worked by assessing the response of rats or mice injected with human urine. If the urine contained hCG, the rat went into heat. In the mid-1900s, scientists upgraded this system by replacing rodents with rabbits; later they used toads. In the 1960s, researchers devised a way to detect hCG without animal sacrifice. By adding a urine sample to a slide full of hCG antibodies, scientists could discern whether hCG was present by observing the antibodies’ response (or lack thereof). If they met hCG, the hCG antibodies reacted, resulting in a sort of muddy ring on the slide. In the next decade, scientists moved beyond the quantitative hCG testing and learned how to test for precise hCG levels with a blood test.
Companies scrambled to introduce the first home test to the market, and consumers could purchase the first “early pregnant test” by 1977, for around $10 (no small price - this would be equivalent to about $41 today). It was a crude version of its more convenient descendants – a far cry from the simple “pee on a stick” undertaking. The initial product “consisted of a test tube, two droppers, and a plastic tube-holder fitted with a special mirror to reveal the results from the bottom of the tube.” Remembering the ordeal, one woman explained that the test would have been simple for anyone with previous lab training and experience. Consumer Reports described: “to use the EPT, a woman must follow a nine-step procedure that permits ample opportunity for error.”
These cumbersome initial home pregnancy tests carried a certain stigma. For decades, the notions that 1) a woman would need to know she was pregnant, and 2) would prefer to know she was pregnant as soon as possible, were simply implausible in a culture that condemned both single mothers and women who opted not to have children. In the twenty-first century, these ideas are (mostly) foreign. Women from all different walks of life expect to be able to learn about pregnancy at their own behest.
Home pregnancy tests offer women the chance to discover and experience a life-changing turn of events in their own bathrooms, on their own terms. And yet, as is true of so many forms of knowledge, more information can sometimes impose unforeseen suffering. The early self-knowledge furnished by home pregnancy tests has led more women to be aware of miscarriages they would not otherwise have known about. I think this phenomenon helps account for my first-trimester stress with my previous pregnancy – I found out I was pregnant so early, when miscarriage rates are highest, that it compounded my anxiety about loss.
Whether a pregnancy comes to light earlier or later, miscarriage is a concern for almost every woman who becomes pregnant. There are a handful of resources that explicate miscarriage risk, and I became intimately familiar with some of them in 2015 (during my first pregnancy). If you absolutely need to look, the “miscarriage odds reassurer” at datayze.com uses day-by-day miscarriage statistics based on the cumulative data from more than 50,000 patients in five peer-reviewed studies to spit out heartening facts about the likelihood of not miscarrying. In the interest of optimism, I tried my best to avoid this kind of thing with this pregnancy. Unfortunately, there is virtually no way to predict or prevent a miscarriage, so women’s time is better spent pondering other matters. For me, looking into hCG and pregnancy tests made for a good distraction.
 M. Boycott and I. W. Rowlands, “The Biological Nature And Quantitative Variation Of The Gonadotropic Activity Of Pregnant Women’s Serum,” The British Medical Journal 1, no. 4037 (1938): 1097–1100.
 G. D. Braunstein et al., “Serum Human Chorionic Gonadotropin Levels throughout Normal Pregnancy,” American Journal of Obstetrics and Gynecology 126, no. 6 (November 15, 1976): 678–81; O. A. Kletzky et al., “Dynamics of Human Chorionic Gonadotropin, Prolactin, and Growth Hormone in Serum and Amniotic Fluid throughout Normal Human Pregnancy,” American Journal of Obstetrics and Gynecology 151, no. 7 (April 1, 1985): 878–84.
 Sarah Leavitt, “‘A Private Little Revolution’: The Home Pregnancy Test in American Culture,” Bulletin of the History of Medicine 80, no. 2 (2006): 318, 322, 324, doi:10.1353/bhm.2006.0064.
 Ibid., 326, 336.
 Ibid., 326, 329, 338.