Miscarriage refers to a pregnancy that ends, on its own, within the first 20 weeks. Miscarriage is the most common type of pregnancy loss, with an estimated 10-25% of all pregnancies ending in miscarriage. The majority of miscarriages occur during the first 13 weeks, also known as the first trimester. Miscarriages can occur for a variety of reasons, but the most common cause is fetal chromosomal abnormalities. The loss of a pregnancy after 20 weeks is not considered a miscarriage.