It is thought that most early miscarriages are caused by a one-off chromosomal fault.

http://www.patient.co.uk/health/miscarriage-and-bleeding-in-early-pregnancy

What causes miscarriage?

It is thought that most early miscarriages are caused by a one-off chromosomal fault. This is usually an isolated genetic mistake, and rarely occurs again. Such genetic mistakes become more common when the mother is older - that is, over 35 years old. This means women aged over 35 years who are having children are more likely to have a miscarriage.