I'm 4 or 5 days late for my period, i took a pregnany test this morning it was negative i've calculated to be about 19 maybe 18 days past conception?

Maybe not pregnant: Can retest in 4-5 days. A urine pregnancy test done correctly will give a reliable result 3 weeks after sex (or when conception might have happened) or 1 week after missing a period. If done sooner, a positive result is likely real, but a negative result may be due to testing too soon, and therefore lead to a repeat test. If a woman gets her normal regular period, then she is not pregnant.