How Pregnancy Tests Work

Testing for pregnancy has a long history. In ancient Egypt, for example, women would urinate on grain and pregnancy would be predicted based on the speed of growth. (This doesn’t, to my knowledge, work.) Most of the time, before the modern day, people knew they were pregnant when they missed a period. 

In the 1920s, the hormone hCG was discovered as a marker of pregnancy. hCG is produced after a fertilized egg attaches to the uterine wall, which usually happens 6 to 10 days post-conception. If the pregnancy is developing normally, the amount of hCG increases over time, roughly doubling every few days. 

The discovery of hCG made it possible to test for pregnancy, although initial testing involved injections into a live rabbit, so it wasn’t very user-friendly. By the 1970s, home pregnancy tests (no rabbits) became available. These early precursors to our modern tests worked basically the same way, but they were much less sensitive, meaning that they could not detect pregnancy as early as the current crop of tests.

So, when it comes to modern pregnancy tests, how do they work, and when should you take one? Let’s dive into the data on some of the most common questions.

How do pregnancy tests work?

Modern pregnancy tests, like the older ones, work by detecting hCG in urine. 

Pregnancy tests are a type of lateral flow test (like COVID tests!). Here’s how it works: If you are pregnant, your urine contains some hCG. When you pee on the stick, the urine flows up through tests and reacts with chemicals in the test. If the test detects a concentration of hCG, it will turn a color (which you’ll usually see as a line or two) or it will show a positive or negative result on a digital display (such as the helpful words “Pregnant” or “Not Pregnant”). 

The sensitivity of these tests — their accuracy — depends on what concentration of hCG they require to show a positive result. That understanding is key to answering many of the specific questions you may have, which I will run through below.

When can I take a pregnancy test?

The earlier you take the test (relative to your ovulation date), the more likely you are to get a false negative result. hCG isn’t produced until the fertilized egg implants. hCG levels double every two to three days after implantation, and levels need to be high enough for a test to detect. To reference the graph below: in this example, ovulation occurs on day 14. The egg may be fertilized on day 15 and then take 6 to 10 days to implant. If implantation occurs on (say) day 23, it may be one or two days after that before levels are high enough to detect.

In this scenario, if you tested on day 8 after ovulation, you would not get a positive test, even if the egg had fertilized and was on the way to implantation.

To be clear: taking a pregnancy test isn’t contraindicated — it has no medical risks. So you can test whenever you want. But you should be aware that even a very sensitive test can give you a false negative if taken too early. 

The other issue to consider: unfortunately, very early miscarriages are quite common, and if you detect a pregnancy very early, some of those early detections will not result in a viable pregnancy and, perhaps, might never have been detected at all if you waited to test. (These types of pregnancies are often referred to as “chemical pregnancies.”) Again, this is very much a question of preferences and emotional approach.

Are some tests better?

Some tests are more sensitive than others. The sensitivity of these tests is measured based on the concentration of hCG that they can detect. Several reviews, including this one, from 2014, test commercial products by exposing them to different concentrations of hCG and seeing whether they are able to detect the results.

The more expensive “early response” tests are often able to detect hormones at a level that would be possible perhaps four days before a missed period. Other tests are less sensitive, but most commercial tests available now — even quite inexpensive ones — would detect pregnancy by the time of a missed period. 

Tests also vary by whether they are digital or rely on lines, whether the handle is plastic, and other design features. These are really about taste.

My husband’s primary approach when buying literally anything is to ask, What does Wirecutter recommend?

What time of day should I take my test?

Typically, in the morning, with your first pee. The reason is that (for most people) it’s the time you’re most dehydrated, so your urine is most concentrated. The higher concentration increases the chance a pregnancy will be detected.

There isn’t anything special about the morning other than the concentration factor, so if there is another time you’re more dehydrated, you could do that. Don’t get dehydrated on purpose, though! Hydration is important.

At some point, hCG concentrations are high enough that it doesn’t matter when you take the test — this morning advice is mostly relevant if you’re testing very early.

Can my pregnancy test be wrong?

False positives are possible, but they are rare. Generally, a positive result, even a faint one, means a pregnancy. If the pregnancy is progressing normally, tests taken on later days should have a more strongly colored line. 

False negatives are much more common, especially if testing early, because hCG levels may not yet be high enough to detect. 

Is a blood test better?

Pregnancy tests using blood are more sensitive; they can usually detect pregnancy almost as soon as the egg has implanted and the placenta starts producing hCG. In addition, these tests can provide a detailed hCG level, rather than just a pregnant/not pregnant indicator.

These tests are involved and it’s unclear the value for most people trying to conceive, but your doctor may choose to do one, especially if you are undergoing fertility treatment.

Could a pregnancy test tell me if something is wrong?

This is a tough question, and of course gets to the fears that are often with us in this process.

Generally speaking, no. A pregnancy test itself would not tell you anything directly about an issue with a pregnancy. Detailed information on hCG levels are informative — for example, hCG tends to be low in an ectopic pregnancy, and a slow growth in hCG can be a signal of a non-viable pregnancy. However, to get this level of detail, you would need a blood test. 

Do pregnancy tests expire?

Yes. Pregnancy tests have a shelf life of two to three years. Over time, the antibodies in the test that react with hCG decay and will not work as well. It’s not like expired food, obviously. Using an expired pregnancy test isn’t going to make you sick, and it’s not going to explode as you pee on it or anything. But over time, the accuracy goes down.

The way in which the accuracy goes down means the test will be more likely to show a false negative. But if you test and you get a positive result, that probably is a true positive.  

The bottom line

• Pregnancy tests work by detecting hCG in urine. hCG is produced once the fertilized egg implants, and it increases rapidly after that. The earlier you take the test (relative to your ovulation date), the more likely you’ll get a false negative result. 
• It is a personal decision how early you choose to test. Most commercial tests available now would detect pregnancy by the time of a missed period.
• The best time of day to take a pregnancy test is in the morning, with your first pee. The reason is that (for most people) it’s the time you’re most dehydrated, so your urine is most concentrated. 
• False positives are possible, but they are rare. Generally, a positive result, even a faint one, means a pregnancy.

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