can 12 hours make a difference in a pregnancy test?

When the Pregnancy Test Positive Line Shows up Later
After getting a negative result on a home pregnancy test, it's fair to conclude that you're not pregnant. However, if you happen to glance back at the test later in the day, you may be surprised to see that a positive line has magically appeared. There are a few reasons why this can happen.
Evaporation Lines
The instructions on most pregnancy tests will advise you to read the results within a certain window of time. This usually ranges between a couple of minutes up until 10 minutes later. If you see a positive result beyond this time frame, you may be left second-guessing the results.

However, the false-positive reading, in this case, is due to something called an evaporation line. Evaporation lines are not indicative of a true positive result.

Evaporation lines happen when the urine that was on the test area starts to dry. The chemical composition of the urine sample changes due to evaporation. As a result, the test may start to display a positive line. After the allotted time has passed, you should discard the pregnancy test to avoid confusion.
Faint Test Lines

If you follow all of the instructions correctly, you may notice a thin, faint test line that appears within a few minutes. It can seem at first that the line is too faint to count, but after 10 minutes, it darkens. There are several possible explanations for this line:

Chemical pregnancy: When implantation of a fertilized egg happens, hCG is produced, even though that egg may not be viable. Chemical pregnancies are the most common cause of early miscarriage.1 Without taking a pregnancy test, the miscarriage may easily be mistaken for a regular period.
Lower sensitivity test: You might need a test with higher sensitivity levels so that you can get accurate results earlier in the pregnancy. Look for "early results" indicated on the test label.
Too early in your cycle: If you test earlier than 10 to 12 days past ovulation, the hCG hormone may not be high enough to produce more than a very faint line. The best time to test is after your period is late, or 2 weeks after you ovulated.2
Too much fluid: You can dilute your urine sample if you are urinating too frequently or drinking too many liquids—this is why first-morning urine samples are recommended for pregnancy testing.3

If you are still unsure about pregnancy test results, try waiting a day or two to take another test. Always use a sample from your first-morning urine (which is more likely to have higher concentrations of the pregnancy hormone, hCG).3
A Word From Verywell

It can very stressful to see conflicting results on a pregnancy test, such as faint lines or evaporation lines. If you're unsure about your results, retest in a couple of days or talk to your doctor about a follow-up blood test for confirmation. Following the directions on your pregnancy test helps avoid the confusion or disappointment of a false positive.