Pregnancy tests, in some form or another, have been around for centuries — even longer than we’ve known about hormones. In the 1300s, people urinated on barley seeds, and in the 1920s and ‘30s, they injected the urine of possibly pregnant women into mice. Thankfully, the pee-on-a-stick and blood-sample-based methods that we rely on nowadays are not only more accurate, but they’re also quicker, easier, and more accessible.
But how does the modern-day pregnancy test work? When should they be used and how useful are they? What are the different options for at-home pregnancy tests? When it comes to ease and accuracy, are all options created equal?
Below, we’re covering what the science and research say.
What’s the basic science that pregnancy tests rely on?
Just as hormones change dramatically across the menstrual cycle, they also change with pregnancy, which may not be so surprising given all of the different processes that pregnancy involves.
Here’s what we know: When sperm meets and successfully fertilizes an egg, the fertilized egg needs to make its way to the uterine lining and dig itself deep for a pregnancy to actually begin. Once this fertilized egg (embryo) successfully implants, the cells surrounding it (which eventually form the placenta) quickly begin releasing a hormone called human chorionic gonadotropin (hCG). And when we say quickly, we mean it — during early pregnancy, hCG concentrations increase by 50% roughly every day. Because the presence of hCG means the presence of a developing embryo, high hCG is an accurate indicator of pregnancy (more on accuracy below).
Besides being used for pregnancy tests, hCG has some pretty cool biological functions. Some work suggests that hCG acts as a “shield” for the developing embryo, protecting it from the mother’s immune system attacking what it considers a genetically foreign “invader.” The hormone also makes sure that a structure called the corpus luteum can secrete high levels of estradiol and progesterone, which are two mega-important reproductive hormones that, among other things, ensure the maintenance of the uterine lining and a developing pregnancy.
There’s one less cool aspect of the hormone that’s also worth noting: hCG concentrations may be linked to symptoms like excessive nausea and vomiting during early pregnancy.
How do pregnancy tests work?
Pregnancy tests work by measuring the amount of hCG in urine and determining whether it falls below or above a certain predetermined threshold — with the idea being that hCG levels at those thresholds are only observed when an embryo has implanted and is developing (but more on false readings below!). This threshold differs for different tests, but generally, the tests detect hCG in urine when hCG concentrations are between 10 mIU/mL and 25 mIU/mL.
What’s the actual chemistry behind hCG detection? If there is hCG in the urine sample that is tested, it binds to anti-hCG antibodies (little proteins) inside the test and triggers a color change in the test line (and the word “pregnant” appears on the screen if the test is digital).
Tests with lower cutoffs are able to detect pregnancy sooner than tests with higher cutoffs, simply because earlier in pregnancy the embryo has not made enough hCG to be detected at the higher ones. Typically, hCG can be detected in urine nine days after fertilization of an egg. Typically, a test with a threshold of 25 mIU/mL can detect pregnancy five days before a missed period, and a test with a threshold of 10 mIU/mL can detect pregnancy six days before a missed period. That being said, the accuracy of a test goes up the longer you wait to take it. Though tests with a threshold of 10 mIU/mL can detect pregnancy six days before a missed period, most are only about 68% accurate on that day. The accuracy jumps to greater than 89% five days before a missed period, and to >97% four days before a missed period.
When and how should I take a pregnancy test?
Different pregnancy tests will come with different instructions, and it’s important to follow those instructions closely (up to 30% of people don’t and this may lead to inaccurate results). For example, while some test manufacturers may suggest you test a few days before the anticipated day of your next period, others suggest waiting until your missed period. If your periods are irregular (making it hard to figure out when your next period would have started), you can track ovulation and luteinizing hormone (LH) surges and use that information to test instead. Start testing about two weeks after ovulation.
You can technically take the test at any time of the day, but it’s best to take it in the morning using your first void (aka your morning urination), which will have the highest concentrations of hCG .
What are my different options for pregnancy tests?
Like ovulation prediction kits (OPKs), there are two main choices you can make when deciding which pregnancy test is best for you:
Collecting your sample
You have two options: strip-based tests and midstream tests. The chemistry for these two is exactly the same, so the “best” option comes down to user preference. Midstream tests are more popular, but it’s unclear whether that’s because they’re easier to use or because that’s traditionally what we see in TV shows and movies when pregnancy tests are used.
- Strip-based tests: You collect your urine in a cup and dip a strip into the urine for a specific amount of time.
- Midstream tests: You pee directly on the test.
Interpreting the results
Because of the technology and additional materials that go into digital tests, you can expect to shell out more money for them as compared to analog tests. What the best option looks like for you depends on how much you want to spend and how much guesswork you want to take out of interpreting test results.
- Analog tests: These require you to do some interpreting when it comes to your test result. With analog tests, you’ll typically have to look for the presence or absence of a second line (present = pregnant; absent = not pregnant) in addition to the control line. Though most people don’t report issues interpreting analog tests, some do, or may not be comfortable interpreting them.
- Digital tests: On the other hand, digital tests don’t leave room for misinterpretation: They’ll display your result as “pregnant” or “not pregnant.”
How accurate are pregnancy tests?
In a nutshell, really accurate. If you’re pregnant and are taking a test on the day of your missed period or after, there’s over a 99% chance that the test will detect that pregnancy. Unless the test you’re using has been subject to a product recall, it’s almost certain that it’s able to detect hCG at the test’s given sensitivity (i.e., if your test says it can detect hCG concentrations at or above 10 mIU/mL in your urine, it almost certainly does). Something worth keeping in mind, though, is that these are manufacturer claims, and some academic studies have found these accuracy percentages to be slightly lower.
False positives and negatives
Like any other test, there are false positives that indicate you’re pregnant when you in reality are not, and false negatives that indicate you are not pregnant when you are (both of which are extremely rare).
False positives can happen when hCG is elevated, but not because of a current pregnancy:
- Some fertility treatments involve hCG injections that result in elevated hCG, but the system will usually be clear of the excess hormones a few weeks after the injection.
- Certain tumors produce elevated hCG (actually, this is why hCG tests were developed in the first place — to detect these types of tumors).
- In some cases, it’s possible that elevated hCG is still present in urine after miscarriage
False negatives can happen when hCG is not detected by pregnancy tests, despite a current pregnancy:
- Testing too early, before enough hCG has accumulated to be detectable by pregnancy tests
- If you’re eager to test before your missed period (or aren’t sure when that is) and get a negative read, it’s best to follow up with a second test a couple of days later.
- Testing with urine that is too diluted. This is why manufacturers typically suggest testing in the morning with your first urination of the day.
All pregnancy tests work by detecting a hormone (hCG) that starts getting produced in high quantities during early pregnancy. Tests with lower sensitivities will be able to detect pregnancy sooner, but beware of starting to test too soon — people who are pregnant but test before hCG has time to rise sufficiently will get false-negative readings.
Because all tests are similar in terms of accuracy (when instructions are followed correctly), the decision to use strip versus midstream and analog versus digital readouts all comes down to personal preference.
And… just for fun: The history of the modern-day pregnancy test
Though pregnancy tests are accessible and commonplace these days, that wasn’t the case until about 40 years ago. In the 1920s, two German scientists learned that injecting sexually immature mice with urine from pregnant people would make those mice ovulate, while urine from non-pregnant people wouldn’t have the same effect. While these tests were accurate, they were expensive, time consuming, and could only be performed by labs that could accept shipments of urine through the mail. As a result, they weren’t an option for most people. When pregnancy tests began using frogs from the ‘40s through the ‘60s, tests were cheaper, but still limited to labs that shipped urine — and people had to rely on their doctors to get tested.
In the ‘60s, scientists began developing pregnancy tests that didn’t require the use of live animals and could be performed in doctors’ offices. They could also detect pregnancy only 2-3 weeks after conception (aka after the first day of a missed period or later). These tests laid the groundwork for the ones we know and use today.
By the 1970s, pregnancy tests that people could take in their own homes became available and could be purchased by anyone who wanted to test. But there was one small (well, big) caveat: The whole process for these early pregnancy tests had about ten steps, and it could take hours. (Oh, and it probably didn’t help that several doctors went on the record saying that people performing these tests would be too emotional to do it correctly.) It wasn’t until the 1980s (1988, to be exact) that modern-day pregnancy tests — you know, ones that have only one step and don’t take up a big chunk of your day — were approved by the FDA and hit shelves.
Fun (and frustrating) fact: The first at-home pregnancy test patent was issued to Margaret Crane, a 26-year-old cosmetics product designer at a company called Organon. Organon initially rejected her idea, but her prototype was eventually chosen for development. Though Crane was instrumental in bringing pregnancy tests to our homes, she has been rarely credited (and was never compensated!) for her involvement.
If reading this has piqued your interest in learning more about hormones, the Modern Fertility Hormone Test is an easy way to get insight into your unique hormone levels — and help you track changes over time. Checking in with your hormones brings you one step closer to becoming an expert about your own reproductive health (and we’ll keep sharing articles like this to up your fertility knowledge in general!).
This article was written by Talia Shirazi and medically reviewed by Dr. Nataki Douglas, the Chair of the Modern Fertility Medical Advisory Board.