The condition of having an abnormal number of chromosomes (extra copies or missing copies of specific chromosomes), or having chromosomes with missing or extra pieces.
Chromosomal abnormalities are the culprit behind most first-trimester miscarriages but do not always lead to miscarriage. For example, Down Syndrome is a fairly common chromosomal abnormality that results from having three copies of chromosome 21. Other chromosomal abnormalities might result in stillbirth or neonatal loss, such as trisomy 18 (three copies of chromosome 18).
In early miscarriages, a baby with chromosomal errors may stop developing because vital genetic information is missing or because the chromosomal arrangement causes the mother's immune system to terminate the pregnancy. Researchers believe that chromosomal abnormalities are behind 60 to 70% of first-time miscarriages. In these cases, the error is usually a random anomaly and the woman will go on to have a normal subsequent pregnancy.
