Why do you have to take a pregnancy test three days after your missed period

  Hi I was wondering why your not suppose to take a pregnancy test until three days after your missed menstrual cycle.  I took a pregnacy test today which was the day that I am due but I never know because my cycles are screwed up I may not start for a week later.  Well anyway it came back negative how much can I rely on this.  I was so happy to see it come back negative and my husband also.  How long does it take the HCG to get into your urine.  I started excersising again today and wanted to know what my percentages are that when I take another one in a week will it still come back negative.  My story is I had sex on the seventh of February and started to bleed on the 12th of february for a couple of days started out reall heavy but then I didn't even know until I wiped myself.  I am not due to start my period until the 21 which is today but I jumped the gun and bought a pregnancy test today.  My breast are real sore and are starting to get red lines through them and look like they are starting to bruise.   I have also been pretty nauseas.  what are my chances that the next test will be negative.  Thank You

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Dear Melissa:
A pregnancy test becomes positive because enough hCG is being cleared from the body to give a concentration in urine high enough to cause the test kit reagents to give a color reaction. hCG is produced from before implantion.The amount that appears in the urine depends on the amount produced by the trophoblast (varies from pregnancy to pregnancy) and the individual metabolism of mother who is clearing the protein from her blood into her urine. Eventually, in every pregnancy, there will be enough hCG to give a positive test. Three days following the missed menstrual period which is 17 days following ovulation in the standardized cycle, the test is positive in almost every woman.
When a woman has an irregular cycle, there are few rules to guide when one should test. There are no statistics to cite the likelihood a test will be positive or negative based on symptoms.
When a test is negative twice, one week apart and/or 21 days from an isolated exposure, you are not pregnant.
Keywords: urine pregnancy test
This information is provided for education purposes and is not a medical consultation. If you have specific questions, please ask your physician.