In more than half of embryos miscarried in the first 13 weeks of pregnancy, the embryo has chromosomes that are not normal. These chromosomal problems may happen because of problems as the embryo grows and its cells make copies of themselves. It is also possible for chromosomal problems to happen because of a problem with a parent's genes. However, this is more likely to happen in women who have had other miscarriages, or if one of the parents has a child or other relatives with birth defects. Genetic problems are more likely to happen with older parents; this may be why miscarriages are more common in older women.