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The Modern guide to home pregnancy tests: How they work and when to test

The Modern guide to home pregnancy tests: How they work and when to test

10 min read

This article was last updated June 16, 2021.

Pregnancy testing, in some form or another, has 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 home pregnancy tests that we rely on nowadays are not only more accurate, but they’re also quicker, easier, and more accessible.

Below, we’re covering what the science and research say, explaining why the Modern Fertility Pregnancy Test is a great option, and answering the following questions:

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 what’s happening to your body in early pregnancy:

  • 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), which is often called the "pregnancy hormone."
  • 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 successful implantation and the presence of a developing embryo, high hCG is an accurate indicator of pregnancy (more on pregnancy test accuracy below).
What’s happening to your hormones in early pregnancy?
From hormones during implantation to hCG levels in early pregnancy, here’s what’s happening in your body after you conceive.

How do pregnancy tests work?

An hCG pregnancy test works 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. (The Modern Fertility Pregnancy Test uses a 10 mIU/mL threshold.)

Tests with lower cutoffs (like ours) 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. hCG can usually be detected in urine nine days after fertilization of an egg. Typically, an hCG pregnancy 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. But pregnancy test accuracy goes up as you get closer to your missed period. (See the chart below.)

Average hCG in early pregnancy
Average hCG in early pregnancy

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).

When and how should you take a pregnancy test?

Different pregnancy tests will come with different instructions, and it’s important to follow those instructions closely to have the most accurate pregnancy test possible (up to 30% of people don’t follow instructions and this may lead to false readings).

  • 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 urine), which will have the highest concentrations of hCG.

What are the different types of pregnancy tests?

Like ovulation prediction kits (OPKs), there are two main choices you can make when deciding which pregnancy test is best for you: strip-based tests versus midstream tests and analog versus digital tests.

Collecting your sample

The chemistry for strip-based tests and midstream tests is exactly the same, so the "best" pregnancy test option really 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 (like ours): Your urine stream goes directly on the test — aka you pee right on it.

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 (like ours): 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.”

What is an early pregnancy test?

The "best" early pregnancy test is the most sensitive pregnancy test. That’s because early tests are sensitive enough to detect levels of hCG in urine before the day of your expected period, helping you get an early result that lets you know if you’re pregnant sooner. The Modern Fertility Pregnancy Test is an early hCG pregnancy test that you can use five days before your missed period for accurate results. And speaking of trustworthy pregnancy test results…

How accurate are pregnancy tests?

In a nutshell, really accurate. If you’re pregnant and taking on or after the day of your missed period, there’s over a 99% chance that the test will detect that pregnancy — the same is true about the Modern Fertility Pregnancy Test. 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, like ours does, 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.

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 and negatives (and how medications can impact results)

A false positive pregnancy test result can happen when hCG is elevated, but not because of a current pregnancy:

  • Some fertility treatments involve hCG hormone 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

A false negative pregnancy test result 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.

And… just for fun: The history of the modern-day pregnancy test

Though home pregnancy tests are accessible and commonplace these days, that wasn’t the case until about 40 years ago.

Pregnancy tests 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 positive pregnancy test results were accurate, the tests 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.

Pregnancy tests in the 1940s

When pregnancy tests began using frogs from the ‘40s until the ‘60s, tests were cheaper, but still limited to labs that shipped urine — and people had to rely on their doctors to get tested.

Pregnancy tests in the 1960s

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.

Pregnancy tests in the 1970s

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.)

Pregnancy tests in the 1980s

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.

The bottom line on home pregnancy tests

All pregnancy tests work in essentially the same way: by detecting a hormone (hCG) in your urine that starts being produced in high quantities during early pregnancy. Tests with lower sensitivities (like ours!) will be able to detect pregnancy sooner — but accuracy increases as you get closer to the first day of your missed period. (People who are pregnant but test before hCG has time to rise sufficiently might get false-negative readings.)

The decision to use strip versus midstream and analog versus digital readouts all comes down to personal preference and how much you want to spend. The Modern Fertility Pregnancy Test uses the same reliable science that all tests use — but is more affordable than the leading pregnancy tests, based on the average price of the top 5 pregnancy tests in retail stores by sales volume. (Source: Nielsen retail sales data in 2021.) It’s also pretty cute.

How the Modern Fertility Pregnancy Test works
The Modern Fertility Pregnancy Test is 99% accurate at detecting pregnancy from the day of your missed period — plus, it’s more affordable than leading pregnancy tests.

Because we’re hormone nerds (and proud of it!), we offer a few more ways to understand the hormones most important to reproductive health.

  • If you’re trying to conceive or just want to get in sync with your cycle, the Modern Fertility Ovulation Test tracks your LH at low, high, and peak levels to help you find your 2 most fertile days, predict ovulation, and get ~in sync~ with your cycle. You can use the Modern Fertility App to track your results from both the Pregnancy Test and Ovulation Test and know when to test for pregnancy.
  • The Modern Fertility Hormone Test measures the same fertility hormones that blood tests at a fertility clinic would (for a fraction of the price) to help you understand how your fertility is changing over time. The test gives you insight into the number of eggs you have and can help you identify red flag issues — like polycystic ovary syndrome (PCOS) or thyroid conditions — that could affect your reproductive health down the line.
  • Our Modern Fertility Prenatal Multivitamin, which helps you prep your body now for pregnancy whenever. It's designed to support your body before, during, and after pregnancy with 12 essential, doctor-backed nutrients. It comes in a sustainable monthly subscription with a (beautifully blue!) reusable glass jar — meaning no more forgotten refills or empty plastic bottles.

This article was written by Talia Shirazi and medically reviewed by Dr. Nataki Douglas, the Chair of the Modern Fertility Medical Advisory Board.

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