For some women, HCG (harmone that causes pregnancy test to show + or -) builds up slowly.
so if the test says negative, and if she did get her period for another week or so, re test.
if she is a month pregnant. any test Would show a Positive result.
yes it should show from 1st day missed period
If she is a full month pregnant, it should show the correct sign - especially if she IS pregnant. You get more false - results than false + results overall.
The earliest a pregnancy test will show positive is 2 weeks after ovulation and 4 weeks after the first day of her last period. If she is more than 4 weeks since her last period i would test. Good luck