Chromosomal abnormalities are a type of genetic condition that cause a fetus (developing baby) to have extra or missing chromosomes. The result can lead to medical problems.

Overview and Symptoms

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 it’s present, the pregnancy often ends in miscarriage. If the baby is born, he or she usually dies shortly after birth. Babies who survive have severe intellectual disability and health problems.
  • 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 tests to determine which pregnancies are at high risk for a chromosome abnormality. These tests include blood work and ultrasound, aren’t invasive and pose no risk to the fetus. If the initial test suggests an increased chance of abnormality, you’ll be given more information about additional tests to consider.

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The Department of Obstetrics and Gynecology at BIDMC provides outstanding care for all women throughout their lives, in a friendly, comfortable and safe environment.

Obstetrics and Gynecology