Roughly 50-60% of miscarriages are caused by chromosome abnormalities, baby has too many or too few chromosomes, or defective genes etc. The Ob who did my d&c said it will either be a chrom ab or an infection that I. have. He said he expects it to be a chrom ab, rather than an infection, because it's more common for it to be down to a chrom ab, and I don't have any signs of infection such as listeria, toxoplasmisis (bad spelling!), rubella, chicken pox etc.
If it's a chrom ab then there's nothing wrong with you, and you can't prevent it happening again, and it is unlikely it will happen again. I guess if you had 3 miscarriages or more, then you'd need to go to a specialist to find out if there's a reason why it keeps happening.
I have so many friends who've miscarried and none have miscarried twice. Having one miscarriage is so common, it really it just bad luck. We all have a much higher chance of a successful pregnancy than having another miscarriage.
I didn't have a blood test done, they just sent the embryo/tissue away. I'm hoping it's a chrom ab thing because it means there's nothing wrong that needs treating (such as an infection), and hopefully it won't happen again.