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Why can results of a home pregnancy test appear to change after several hours? #836/8
Why can the results of a home pregnancy test (HPT) appear to change after several hours? I took one this morning with first morning urine and read the results after 3 minutes - it was definitely negative, no sign of a second line. I set it aside, forgot to throw it out. (and ironically, started my period about 30 minutes later, right on time.

I am also on birth control pills (BCPs) and have taken them every day this cycle with no mistakes.  This afternoon, about 6 hours later, there appears to now be a second line indicating a positive result. I checked the test directions and it says not to read results after 10 minutes. Why does my test appear to be positive now and can I assume I am not pregnant since I started my period on time and have all my usual symptoms?

Thank you.  This website is incredibly informative and helpful!

First, thanks so much for your kind words.  We try!

You CAN definitely assume that you are not pregnant.  Some chemical tests are extremely time-sensitive as the instructions for your test suggest.  Isn’t it nice, so very nice, that your negative pregnancy test was confirmed for you by the start of your period?  Just 30 minutes after that negative pregnancy test!


Your experience was so interesting I am going to send this along to the author of the pregnancy testing chapter in Contraceptive Technology, Mary Fjerstad, NP.

“Even non-pregnant women have a little bit of beta hCG in their serum (so presumably in urine as well).  Any beta quant below 5 mIU/m1 is normal for a non-pregnant woman.  If a urine pregnancy test sits around for a long time-past the recommended read-time, even a minute amount of hCG can migrate and turn the test line positive.” [Mary Fjerstad, Nurse Practitioner]


Mary Fjerstad NP, MHS

Clinical Training Director

Planned Parenthood Consortium of Abortion Providers

Key Words:  home pregnancy test, urine, results, negative, period, second reading, positive results, pregnant, birth control pills, chemical test, sensitive, Contraceptive Technology

Posted 8-28-2008, Updated 9-4-2008, Updated 9-17-2008, Updated 10-5-2008 

Robert A. Hatcher MD, MPH
Emeritus Professor of Gynecology and Obstetrics
Emory University School of Medicine
Atlanta, GA

Managing Contraception for Your Pocket 2013-2014

Managing Contraception for Your Pocket 2013-2014

The directors and owners of this website and any publications and information concerning health matters offered here advise a person with a particular problem to consult a primary-care clinician or a specialist in obstetrics, gynecology, or urology (depending on the problem or the contraceptive) as well as the product package insert and other references before diagnosing, managing, or treating the problem.
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