[header] How to determine if you had a miscarriage [title] Understand why miscarriages happen. [step] Miscarriages most often occur in the early weeks of pregnancy. Chromosome abnormalities are the most common cause, and in most cases there's nothing the mother could have done to prevent it. 
The risk of miscarriage drops after thirteen weeks of gestation. By then, most chromosome abnormalities would have already caused the pregnancy to end.