Miscarriage is an unplanned loss of a pregnancy before the 20th week. Many miscarriages happen because the fetus isn't growing as expected.
Chromosomes are responsible for more than 50% of early pregnancy loss. Chromosomes are in every cell of the body. They give directions to the body's genes on how to grow and develop. If there is an unusual number of chromosomes as the embryo divides and grows, it can lead to miscarriage. Problems with chromosomes become more common as a pregnant person gets older.
Sometimes a health condition might lead to a miscarriage. Examples include poorly controlled diabetes and conditions that affects the uterus. But often, the cause of miscarriage isn't known.
About 10% to 15% of known pregnancies end in miscarriage. But the number of miscarriages is likely higher. Many miscarriages happen before a pregnant person even knows about the pregnancy.