table of contents
- A quick refresher on the science of pregnancy tests
- How long do you have to wait after ovulation before taking a pregnancy test?
- What happens if you take a pregnancy test too early?
- Can anything else impact your pregnancy test results?
- Have we always had to wait two weeks to test for pregnancy?
- The bottom line
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If you’ve decided you’re ready to start or grow your family, you may be eagerly anticipating the moment you hold a pregnancy test in your hands and see those lines indicating a positive result. But getting to that point takes some time.
To get the most accurate reading on a home pregnancy test (and avoid a false negative result) it’s recommended to wait two weeks after ovulation before testing. That’s because these tests are designed to detect human chorionic gonadotropin (hCG) in your urine — and your body doesn’t start producing hCG until the fertilized egg attaches to the uterine wall. This is called implantation, and it takes about two-ish weeks after fertilization.
In this article, we’ll break down the science and history of pregnancy tests and what you need to know in order to get the most accurate results when using them.
Here are the most important takeaways:
- Home pregnancy tests work by detecting the level of hCG in your urine. Early in pregnancy, levels of hCG rise rapidly after a fertilized egg implants into the wall of the uterus.
- The “two week wait” refers to the two weeks between ovulation and when your hCG levels are likely high enough to produce a positive result on a pregnancy test. If you test too early, you might get a negative result, even if you are pregnant.
- It’s important to follow the testing instructions on your home pregnancy test to ensure the most accurate results.
A quick refresher on the science of pregnancy tests
The most surefire way to confirm a pregnancy is through a blood test or ultrasound done at a doctor’s office. But before visiting the clinic, at-home pregnancy tests can give you answers with about 99% accuracy (when you follow the instructions correctly).
How exactly do these tiny sticks detect a pregnancy? It has everything to do with the level of hCG in your urine.
Here’s what you need to know about hCG:
- When a fertilized egg implants into the uterine lining, the body starts ramping up production of hCG — increasing by roughly 50% every day during the early stages of pregnancy — in order to nourish the developing embryo.
- Urine-based pregnancy tests compare the levels of hCG in your urine to a predetermined hCG threshold that indicates pregnancy.
- For a positive result, urine generally needs to have about 20 mIU/mL of hCG. Urine that contains anything less than 5 mIU/mL of hCG will show as negative.
How long do you have to wait after ovulation before taking a pregnancy test?
The instructions of most home pregnancy tests recommend waiting until the first day of a missed period since that’s when you’ll get the most accurate results. If you’re tracking your cycle, this means waiting around two weeks (based on a “textbook” 28-day cycle) from the time of ovulation before taking a pregnancy test.
For those who want results as soon as possible, some pregnancy tests are designed to detect a pregnancy as early as six days before your missed period — though results will become more accurate the closer you are to your missed period.
Tracking ovulation and luteinizing hormone (LH) can help you determine exactly when to test for pregnancy — especially if you have irregular cycles.
What happens if you take a pregnancy test too early?
Remember that hCG levels increase rapidly after implantation, which is when a fertilized egg attaches to the uterine lining. Completed implantation is a slow process that can take up to two weeks. If you’re pregnant, but try to test before implantation happens, your hCG levels will still be too low to produce a positive result.
Can anything else impact your pregnancy test results?
One of the biggest factors that impact results from a pregnancy test is not following the testing instructions. An estimated 30% of people have trouble following a pregnancy test’s instructions, which can lead to false readings. But, when used correctly, pregnancy tests will rarely produce an inaccurate result.
That said, there are a few situations where the body will produce high levels of hCG that mimic the levels in early pregnancy and cause a false positive result:
- During some fertility treatments, like egg retrieval and IVF, injectable gonadotropins will increase the production of hCG. In most cases, your body will usually be clear of the excess hormones a few weeks after the injection.
- Certain tumors in the reproductive system, even benign ones, can cause hCG levels to rise.
- A chemical pregnancy or recent miscarriage will elevate hCG levels because implantation occurred. If you get a positive result, then retest a few days later and get a negative one, it’s possible that you’ve experienced a very early pregnancy loss.
Have we always had to wait two weeks to test for pregnancy?
While two weeks might feel like an excruciatingly long time to wait, the pregnancy tests of yesteryear required people to wait even longer:
- Ancient Egyptians figured out a simple urine-based test that required peeing on bags of seeds and waiting a month to see if they sprouted (which modern studies found to be surprisingly accurate). If you’ve ever tried watching grass grow, you can probably imagine how ancient Egyptians felt waiting to know if they were pregnant.
- In the late 1920s, a lab test was created that could accurately detect hCG in a woman’s urine — though it too was far from efficient. The test required a woman’s urine to be injected into five mice, twice a day for three consecutive days. At that point, they would dissect all five mice and inspect their reproductive organs. If their ovaries were swollen and mature-looking, the woman was pregnant. In order to get an accurate result, women needed to wait two weeks after they missed their period: a total of one month after ovulation.
- Since then, tests have gotten more sensitive to hCG. Yet, despite many medical innovations since the 1920s, there are some things science hasn’t been able to hack: The body needs two weeks from ovulation in order to produce high enough levels of hCG for the most accurate pregnancy test result. Perhaps this is a gentle reminder that, despite the fast-paced nature of our world today, some things will always just take time.
The bottom line
It is completely understandable if you find it hard to wait two weeks before giving a pregnancy test a try. But, at the very least, you don’t have to worry about finding the best spot by the window for your bag of pee-seeds or a good hiding place for your box full of mice!
Just remember that until a fertilized egg has implanted on the uterine wall, any pregnancy test will read negative. So waiting those two weeks is critical for you to get the most accurate results.
This article was medically reviewed by Dr. Jennifer Conti, MD, MS, MSc.
Dr. Jenn Conti is an OB-GYN and serves as an adjunct clinical assistant professor at Stanford University School of Medicine.