If you’re trying to get pregnant, you’ve probably experienced the agony of wondering when to take a pregnancy test. Do you really need to wait until the first day of your missed period, or can you test earlier?
There are really two answers to this question: when you can, and when you should.
In theory, you can take a pregnancy test from the very moment implantation took place—between three and 12 days after ovulation. But it’s much better to wait until the first day of your missed period, because testing early carries a greater risk of false positives and false negatives.
First, concentrations of hCG vary among women at the same stage of pregnancy, with some not reaching detectable levels until after the day of the expected period. In these cases, early testing can produce a false negative result. Second, a high proportion of pregnancies end in miscarriage very early, often before the first missed period—but hCG may remain above the detectable threshold for days, giving a false positive result.
How to avoid a false positive pregnancy test
- Test on the day of your missed period and not earlier.
- Repeat the test two days later, regardless of the results. If you get a negative result the first time, it may be because your hCG hasn’t risen enough yet for the test to detect it. If you get a positive result, a second test two days later can rule out a very early pregnancy loss, also known as chemical pregnancy.
What causes a false positive pregnancy test?
- Your LH Peak. Sometimes, hCG can be released from the pituitary at the time of your LH-peak. If you take a pregnancy test during this time, there’s a small chance of a false positive.
- Chemical pregnancy. A high proportion of early pregnancies are miscarried even before the day of your period. Taking a test too early could detect a very early pregnancy that is not viable.
- Perimenopause. Perimenopausal women have somewhat elevated levels of circulating hCG, even when they’re not pregnant. This could lead to a false positive result.
- Infertility treatment. Women undergoing hCG treatment (A.P.L, Pregnyl, Profasi, Pergonal) to stimulate ovulation may also receive false positive results if they test within 10 days of their last injection.
What causes a false negative pregnancy test?
1. Testing too early. In very early pregnancy, not all women produce enough hCG for the test to pick up.
2. Using a test that is not sensitive enough. If you are testing early, you need to use a more sensitive test.
3. Diluted urine. If you drink a large amount of liquid shortly before testing, your urine volume may increase so much that hCG is too dilute to measure.
4. Not testing urine immediately. If the urine collected has been sitting around for too long before testing—usually 30 minutes or more—a false negative result can occur.
What type of pregnancy test is best?
If you’re testing on the first day of a missed period, it doesn’t matter very much which test you use—most currently marketed tests are reliable and will yield a positive result for 98 percent of pregnant women. But if you’re testing early, you should use a more sensitive test—just remember that pregnancy tests are less likely to be accurate when taken before your missed period.
The more sensitive the pregnancy test, the more likely it is to detect pregnancy early. But more sensitive pregnancy tests are also more likely to return false positives.
In a study1 that tested six over-the-counter pregnancy tests, First Response and Answer were the most sensitive. Clear Blue Easy and Walmart Equate tests were the least sensitive.
- Cervinski, M. A., Lockwood, C. M., Ferguson, A. M., et al. (2009). Qualitative point-of-care and over-the-counter urine hCG devices differentially detect the hCG variants of early pregnancy. Clinica Chimica Acta, 406(1-2), 81–85. ↩