How To Prevent Early Miscarriages?

Hi doctor, earlier i had 2 misscarriages both was early misscarriages first end up with 1 week pregnancy and 2nd one end up with 4 week pregnancy at the time of second pregnancy my dctr knew tht i already had a misscarriage but she didn t put me on duphaston when i was confirmed pregnnt tht end with misscariage my fotus not grow.i saw in some of web site duphston will prevent misscarriages my question is when doctr will precribe duphaston in pregnncy and why my fotus is growing both the end?i have one question if i get pregnant again it will happen to me again?how to prevent early misscarriages when i get pregnnt again plz advice me