Pregnancy tests work by detecting the presence of a hormone called human chorionic gonadotropin (hCG) in urine. This hormone is produced by cells that will form the placenta when a fertilized egg implants in the uterus1.
A change in a pregnancy test result from negative to positive after several hours could be due to various factors:
Quality of the Test
Not all pregnancy tests are created equal. Some tests are more sensitive and can detect lower levels of hCG than others. If a test isn’t very sensitive, it may initially show a negative result, but as the urine sample continues to sit on the test, it might eventually turn positive as it has more time to react with the hCG, if present.
User Error
Incorrect use of the test can also lead to a false negative or delayed positive result. For example, if the test wasn’t left in the urine stream for long enough, or if the test was taken too early in pregnancy when hCG levels are still low, it could initially show a negative result3.
Biological Reasons
In some cases, a “late” positive result could be due to a phenomenon known as an “evaporation line.” This happens when urine evaporates off the test strip after a certain period of time, leaving a faint line that can be mistaken for a positive result4.
Next Steps
If a woman’s pregnancy test result changes from negative to positive after several hours, it’s recommended she take another test to confirm the result, ideally first thing in the morning when urine tends to be more concentrated with hCG. She should carefully follow the instructions that come with the test, ensuring she leaves the test in her urine stream for the correct amount of time5.
It’s also important to read the result within the timeframe specified in the instructions. If a test shows a positive result, or if menstrual periods continue to be missed, it’s recommended she schedule an appointment with a healthcare provider to confirm the pregnancy6.
Finally, while home pregnancy tests are generally accurate, they are not infallible and can sometimes give false results. Therefore, any concerns or unusual results should always be discussed with a healthcare provider.