From Robin Elise Weiss, LCCE,
Your Guide to Pregnancy & Childbirth.
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A. A urine pregnancy test works by measuring the amount of hCG (pregnancy hormone) in your urine. Typically you need to have missed your period for these to be accurate, particularly a positive result. That said, if you have a positive a couple of days before this missed period, hCG has been detected and you are pregnant.

As the tests become more sensitive, more and more women will have positive results earlier. That leaves the problem of taking a pregnancy test prior to your missed period and getting a negative result.

If you get a negative result and you have tested prior to your missed period, you will need to wait a couple of days and retest.

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