Being pregnant for two times now & miscarriage occurred at week 5 in both. What's the possible reason for that??

Chromosome problems. About 50 % of all miscarriages are caused by either an inadequate number or configuration of the fetus' chromosomes. If the miscarriages persists, ask your GYN MD to check you for an endometrial or other infection and have the fetal tissue tested as well. The other most common cause is a cervix that opens prematurely, usually in the 2nd trimester.(after 12 weeks) of pregnancy.