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Most of the urine pregnancy test kits usually start showing positive results after about 20 days post-conception (roughly corresponding to about a week after your missed periods). Sometimes urine pregnancy tests can keep showing you false negative results during early pregnancy as the kits are calibrated ones, so unless the hormone levels in urine reach a certain level, they do not show positive results. The symptoms (delayed periods, tender breasts & nipples, nausea, abdominal cramps, brown discharge) could be suggestive of possibility of pregnancy. If pregnancy is suspected, look out for development of any other new symptoms. Wait and watch for your periods. If you do not get your periods within 10 days of expected period dates, or if you get abnormal periods, or if you develop symptoms suggestive of pregnancy, you should consider visiting your gynecologist for thorough examination (to confirm pregnancy related changes/signs in body) and blood/serum HCG level test (to confirm/rule out pregnancy accurately). Blood HCG levels can show accurate results as early as about 11 days post-conception. Take adequate rest. Drink plenty of water. Maintain healthy diet. You may also consider taking a repeat urine pregnancy test (on an early morning first urine sample) after 7-10 days to see if you get positive results, or get it confirmed by a blood HCG test (avoid lot of fluid intake or caffeinated beverage intake in the previous night before test).
Take care.