My wife missed period 18 days .i do test but result is negative then when i do the test?

Tests are accurate: A urine pregnancy test, done correctly, gives an accurate result 3 weeks after sex (3 weeks after when conception might have occurred), 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 thus lead to a repeat test. If worried, one can repeat a test after another week.