How Pregnancy Tests Work
Whether it is a blood or urine test, all pregnancy tests, including in-home pregnancy kits, work the same way—by testing for the presence of human chorionic gonadotropin (hCG). hCG is a hormone produced in the placenta after egg fertilization. Its concentration doubles every 2-3 days, peaking around the 8th week of pregnancy.
How Soon You Can Take the Test
Many tests can start to detect pregnancies as early as 10-15 days after conception or 1 day after a missed menstrual period. But since the level of hCG continues to rise, the test becomes more sensitive over time. So on the first day after a missed period, the urine pregnancy test may only detect some pregnancies, but 1 week later it would likely detect most pregnancies. If you tested and the result was negative, and you still have not started your period, wait a few days and test again.
How to Use an In-Home Pregnancy Test
It is important that you read the directions. Not all in-home pregnancy kits are used in the same way. With some kits, you will urinate in a cup and then use a dropper to place a small sample of your urine on the test area. Other kits require you to urinate directly on the test stick. The results are generally ready in under 5 minutes.
In-home pregnancy tests are very accurate if used appropriately 1 week or more after your missed period. However, the results are much less accurate if the test is done incorrectly, if the instructions are not followed, or you use the test too early.
It is important that you follow the directions and understand how to interpret the results. Manufacturers state that in-home pregnancy tests can be used as soon as 1 day after a missed period, but understand that the tests are much more accurate if you wait a week after a missed period.
When an error does occur, more often than not, it is a false-negative—meaning the test says you are not pregnant when you are. If the test result is negative but you are experiencing early signs of pregnancy, it is best to see your healthcare provider. The earlier that you begin prenatal care, the better it is for both you and your baby.
If your home pregnancy test is positive for a pregnancy, it is very likely to be correct. It is extremely rare that a test would give you a positive result if you were not really pregnant.
Other Possible Causes of a Missed Period
The most common cause of a missed period is pregnancy. Some other causes of missed periods include but are not limited to:
- Birth control pills— Some women who use this form of contraception may not have periods. In addition, stopping your birth control pills can lead to 3-6 months of missed or abnormal periods.
- Breastfeeding— Although this can cause missed periods, you can still get pregnant without periods when you are breastfeeding.
- Medications— Certain drugs can cause changes to your monthly cycle. When you are prescribed a new medication, ask about possible effects on your menstrual cycle.
- Illness— Some illnesses can disrupt your period. Once you are healthy, you should return to your normal cycle.
- Eating disorders, malnutrition, and low body weight— These may cause a decrease in the production of estrogen. Inadequate estrogen production can result in your menstrual cycle stopping.
- Excessive exercise— This can disrupt the production of hormones and stop menstruation.
- Menopause— As you near menopause, your periods will become irregular and eventually stop.
American Congress of Obstetricians and Gynecologists
Office on Women's Health
The Canadian Women's Health Network
The Society of Obstetricians and Gynaecologists of Canada
Pregnancy testing. Planned Parenthood website. Available at: https://www.plannedparenthood.org/teens/going-to-the-doctor/pregnancy-testing. Accessed November 16, 2017.
Pregnancy tests. Women's Health—US Department of Health and Human Services website. Available at: http://www.womenshealth.gov/publications/our-publications/fact-sheet/pregnancy-test.html. Updated February 6, 2017. Accessed November 16, 2017.
Taking a pregnancy test. American Pregnancy Association website. Available at: http://americanpregnancy.org/getting-pregnant/taking-a-pregnancy-test. Updated March 14, 2017. Accessed November 16, 2017.
Urine pregnancy test. EBSCO DynaMed website. Available at:http://www.dynamed.com/topics/dmp~AN~T907272/Urine-pregnancy-test. Updated July 7, 2017. Accessed November 16, 2017.
Last reviewed November 2017 by EBSCO Medical Review Board Michael Woods, MD, FAAP Last Updated: 12/10/2015