The major cause of an early miscarriage is probably a genetic (inherited) factor. An estimated 50 to 60 percent of all first trimester miscarriages of a single fetus (not twins or more) show evidence of genetic defects. The most common genetic defect resulting in miscarriage is an abnormal number or structure of chromosomes. If it’s determined that a pregnancy loss is due to a chromosomally abnormal fetus, there’s a greater chance that a subsequent pregnancy will also be abnormal. However, if the chromosomal examination of the fetus is normal, there’s a reduced chance that a subsequent pregnancy will be chromosomally abnormal.