I do believe it is because the concentration of the hCG (hormone? ) is easier to detect in the more concentrated wee. But what tests look for can vary from brand to brand as this US site shows.
Hi,
the Hcg is higher in the first pee of the morning. also i have found that if you drink more before the test it does alter the result. Best thing would be to try it again today and then tomorrow as the Hcg level will increase each day when you are pregnant making it stronger and hence tests being positive. Medication can also sometimes affect the result of your test. I.E Antibiotics etc.
I found clear blue digital tests where it appears pregnant or not pregnant to be the best and most clear.
Good luck!
A nurse friend of mine told me that pregnancy tests can give a negative reading when you are pregnant but don't tend to give a positive reading if you are not pregnant.
I was also under the impression you can not get a false positive from a pregnancy test, the test is *looking* for the pregnancy hormone, it can't find it if it's not there. The only way you might get a "false" positive is in the case of an ectopic pregnancy.