You can take a pregnancy test anytime you’d like to, even if you’re not pregnant! But, as we discuss in this article, there are optimal windows for taking home pregnancy tests to ensure that you get the most accurate results.

Pregnancy tests work by measuring a hormone called human chorionic gonadotropin (hCG). hCG is the first hormone that your body makes after a fertilized egg starts to attach and implant into the wall of the uterus. Women’s bodies start to make hCG about 6 days after fertilization and hCG levels double every 2 to 3 days in early pregnancy.

There are two main types of pregnancy tests - those that measure hCG levels in the urine and those that measure levels of hCG in the blood. Urine pregnancy tests are available over-the-counter and can be done at home. hCG blood tests are used in the clinical setting and are ordered by doctors or midwives. Urine test results are typically available within minutes, while blood pregnancy tests can take several days, and, in some cases, up to a week to result. All positive home pregnancy tests should be confirmed by tests that measure the amount of hCG present in the blood at your doctor's office. 

It’s recommended that women wait to test until the first day of their missed period for optimal results. However, in some cases, this might be too early for there to be high enough hCG levels for a test to register as positive. Because of this, experts recommend that all negative home pregnancy tests be repeated about a week later for confirmation of results. There are some at-home pregnancy tests that are marketed for use as early as a week before one’s period is due, but these tests have a higher rate of false-negative pregnancy test results than traditional home pregnancy tests. 

Home pregnancy tests all work the same way (by measuring hCG), so doctors do not typically recommend certain brands over others. One of the most cost-effective ways to do home pregnancy tests is to buy them at your local dollar store. These pregnancy tests are just a fraction of the price of name-brand pregnancy tests.

Approximately 5% of pregnancy tests are false negatives. The most common reasons for false negatives are if tests are done too early on, before the first day of one’s missed menstrual cycle, or if the test itself is faulty. It’s important to keep in mind that no tests are perfect and, for any pregnancy test, there are risks of both false positives (test showing you are pregnant even if you are not) and false negatives (test showing you are not pregnant even if you are).

The only medications that might affect the results of a urine pregnancy test are fertility treatments. If you have experienced infertility and are working with a reproductive endocrinologist you should discuss how and when to do your pregnancy test(s) to yield the most accurate results.

Breastfeeding does not affect the results of a pregnancy test as the hormones involved in milk production do not cross-react with the pregnancy hormone hCG.

An evaporation line is a faint, colorless line that shows up on a pregnancy test where a positive line should be, caused by evaporating urine. The most common reasons for an evaporation line to appear is if a test is being read far outside of the recommended time frame for interpretation or if a test has been done incorrectly. An evaporation line can be differentiated from a positive test result, in that the lines in positive tests are colored, thicker, and darker than evaporation lines.

A chemical pregnancy is a very early pregnancy loss that can cause one to have a positive pregnancy test followed by a negative pregnancy test. This is usually due to there being a problem during the implantation of a fertilized egg. It’s estimated that up to 50% of conceptions end in chemical pregnancies!

• Read all instructions carefully before taking your pregnancy test. This is especially important if you are using a new type of pregnancy test as the steps involved in testing may differ amongst brands.
• Check the expiration date to make sure your test is not expired.
• Make sure to take the test during the correct time frame after ovulation. In most cases, the earliest you should take a pregnancy test is the first day of your missed period. 
• The best time of the day to take a pregnancy test is first thing in the morning. 
• If you get a negative test result but believe you might be pregnant, you should do another home pregnancy test 5-7 days later to determine whether or not you are pregnant.
• Any home pregnancy test with a positive result should be followed up by a blood hCG level test from your healthcare provider or ob/gyn to confirm that you are pregnant and that you have appropriate hCG levels in your blood.