Pregnancy question- missin period?

if u were pregnant a week before ur period u would miss that period but a pregnancy test would not detect that ur pregnant at that stage so if u tested the first day of ur period u would get a neg result as the hcg levels in ur body is not high enough u would need to go to ur doc and get a blood test if u thought u were pregnant or wait anopther couple weeks and take another test... i found out i was pregnant at 3 weeks by first response preg test...