Chromosomal Abnormalities

The most common differences related to the number of chromosomes include:

• Down syndrome — also called trisomy 21 because it occurs when there’s an extra copy of chromosome 21 — which causes mild to moderate intellectual disability, typical facial features and, sometimes, birth defects. The range of intellectual disability in Down syndrome varies.
• Trisomy 13 and trisomy 18, which happen when there’s an extra copy of chromosome 13 or chromosome 18. If present, the pregnancy often ends in miscarriage. If the baby is born, he or she usually has severe intellectual disability and health problems and survival may be limited.
• Sex chromosome differences. Most people who have an extra or missing sex chromosome don’t know it. Having a different number of sex chromosomes usually doesn’t cause serious problems with development or thinking. Some affected children may have learning disabilities and some affected people have infertility as adults.

In all pregnancies, women are offered screening tests to determine which pregnancies are at higher risk for a chromosomal abnormality. These tests include maternal blood screening and ultrasound; they are non-invasive and pose no risk to the fetus. If the initial screening suggests an increased chance of abnormality, your doctor will discuss further diagnostic tests such as:

• Noninvasive prenatal testing (NIPT) — a highly sensitive blood test that examines fetal DNA in the mother's bloodstream.
• Chorionic villus sampling (CVS) — an invasive diagnostic test performed around 10–13 weeks that samples placental tissue.
• Amniocentesis — an invasive diagnostic test usually performed after 15 weeks that samples amniotic fluid to analyze fetal chromosomes.
• Genetic counseling — essential for families to understand test results, implications, recurrence risk, and available supports.