Chromosomal disorders arise from errors in an entire chromosome rather than mutations in a single gene. These errors occur when the egg and sperm are forming and may be inherited from either the father or the mother.
Perhaps the most familiar chromosomal disorder is Down syndrome, which occurs when a child receives three copies of a certain chromosome instead of two. Other birth defects result when chromosomes are broken, missing, or rearranged.
Unlike single-gene diseases, chromosomal disorders cannot be predicted by screening parents before pregnancy. There are screening tests your doctor may recommend during pregnancy to help determine whether your fetus is at a greater risk for chromosomal disorder.
Prenatal disgnostic tests (CVS or amniocentesis) will reveal whether your baby's chromosomes are normal. Because the risk of chromosome abnormalities increases as women age, doctors will typically recommend prenatal testing for mothers who will be over 35 at the time of birth.