SOme test kit very sensitive,can test it 3 or 4 days before your next period,most test kit can test it at the day you miss your period.
Also depends on the HCG level,some women HCG level is too low to test at period day, can test it again after your period miss couple of days, using morning urien
Also depends on the HCG level,some women HCG level is too low to test at period day, can test it again after your period miss couple of days, using morning urien