13 days in the same room, and then pregnant with white board. Why?

After 13 days of the same room with early pregnancy test paper check white board phenomenon, then check and pregnant, may be because after 13 days of the same room, pregnancy time is too early to cause. General recommendations after a week of menstrual delay with morning urine early pregnancy test paper check, so there is diagnostic significance. In addition, but also in the menstrual period 45 days or so to do B-ultrasound examination, diagnosed as intrauterine pregnancy.