Not seen any heartbeat

hi mommies i just want to ask, i went for a checkup at womens clinic for an ultrasound, im at 7wks2days the doc dont see any heartbeat. this is nt the first pregnancy with no heartbeat. i dont knw why it happen to me twice the same as the previous pregnancy . Im worried if i had a miscarriage again. :/ i dont knw why/what are the causes to have no heartbeat.#pregnancy