Chromosomal abnormalities
Overview
About 1 in 150 babies is born with a chromosomal abnormality (1, 2). These are caused by errors in the number or structure of chromosomes. There are many different chromosomal abnormalities. Many children with a chromosomal abnormality have mental and/or physical birth defects. Understanding what chromosomes are may make it easier to understand the wide range of problems chromosomal abnormalities can cause.
Causes
Chromosomal abnormalities usually result from an error that occurred when an egg or sperm cell was developing. It is not known why these errors occur. As far as we know, nothing that a parent does or doesn't do before or during pregnancy can cause a chromosomal abnormality in his or her child. Sperm and egg cells are different from other cells in the body. These cells have only 23 unpaired chromosomes. When an egg and sperm cell join together they form a fertilized egg with 46 chromosomes. But sometimes something goes wrong before fertilization. An egg or sperm cell may divide incorrectly, resulting in an egg or sperm cell with too many or too few chromosomes. When this cell with the wrong number of chromosomes joins with a normal egg or sperm cell, the resulting embryo has a chromosomal abnormality. A common type of chromosomal abnormality is called a trisomy. This means that an individual has three copies, instead of two, of a specific chromosome.
Diagnosis
Chromosomal abnormalities can be diagnosed after birth using a blood test, or before birth using prenatal tests (amniocentesis or chorionic villus sampling). Cells obtained from these tests are grown in the laboratory, and then their chromosomes are examined under a microscope. The lab makes a picture (karyotype) of all the person's chromosomes, arranged in order from largest to smallest. The karyotype shows the number, size and shape of the chromosomes and helps experts identify any abnormalities.