What Causes Miscarriage?

Hi Doctor, U This is regarding pcos I got married 3 years before and currently my age is 26f.In 2012 I came to know I was having pcos and my periods are irregular.Doctor advised me to take M2tone for 2 months and my period started regular and conceived twice in 2013 but unfortunality I got misscarriage both the times.I had gone different tests like hystroscopy and results are normal. Again from last 4 months my periods are irregular and doctor advised me to take metformim. 1.Can take M2tone sryup along with metformim? 2.I am not sure what s was reason for misscarriage and both I lost at 8th week of pregnancy