I am in the 1st day of the 8th week , is it normal...

its generally know that you should be able to hear the fetal heartbeat by the 7th week or your gestation but it's not always the case 
sometime being not able to hear the heartbeat at 7th or 8th would not mean a miscarriage, especially when you have an abnormal previous menstruation because that would results in incorrect calculation of the gestational age 
 most of the time telling that you have a miscarage  would depends on several factors seen by the ultrasonographist that depends on the measurement of the fetus and its sac and presence or absence of the yoc sac
sometimes your doctor would confirm miscarriage in certain situation such as when the heartbeat was heard but no more or there is falling in B-hcg measurement in addition of the absence of the heart beat
advice 
you should keep taking the folic acid and any other medication prescribed by your doctor and repeat the US next week to be sure of the pregnancy status