It's officially classed as a miscarriage if the pregnancy is spontaneously and unintentionally ended before 16 weeks. In some countries, this is set at 20 months. In Germany, there is no time factor; instead, miscarriage is classed when the baby weighs up to 500g.
Miscarriages are far more common than most people think; around 25% of women will experience one in their lifetime and 90% of all fertilized ova can extremely premature. Usually, these extremely early miscarriages are not noticed because a normal menstrual period follows. Thus, half of all pregnancies end before the woman knows she's pregnant. On average, 15% of all "established" pregnancies end in miscarriage.
The age of the mother is the prime risk factor for miscarriage; the risk being 9% for women between 20 and 24 years of age, over 50% for women over 42, and up to 75% for women aged 45.
From Cassandra-Harmon In Q&A