2nd miscarriage in 8 months...

Had a miscarriage in May at 5 weeks. Felt like forever ttc again. December we got our BFP. But new years eve I started to miscarry. I was 6 weeks. What am I doing wrong?! I have 2 healthy daughters from a previous marriage. My husband has none of his own. I have normal levels of progesterone, I ovulate on time, I'm regular, had an hsg done, an ultrasound on my ovaries, I just don't understand!