Chromosomal abnormalities usually result from an error that occurs when an egg or sperm cell develops. 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.

Egg and sperm cells each contain 23 chromosomes. When they 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 of a specific chromosome, instead of two. For example, individuals with Down syndrome generally have three copies of chromosome 21 (though a small number of cases are caused by chromosomal rearrangements).

In most cases, an embryo with the wrong number of chromosomes does not survive. In such cases, the pregnant woman has a miscarriage. This often happens very early in pregnancy, before a woman may realize she’s pregnant. More than 50 percent of first-trimester miscarriages are caused by chromosomal abnormalities in the embryo.

Other errors can occur before fertilization. These errors can alter the structure of one or more chromosomes. Individuals with structural chromosomal abnormalities usually have the normal number of chromosomes. However, small pieces of a chromosome (or chromosomes) may be deleted, duplicated, inverted, misplaced or exchanged with part of another chromosome. These structural rearrangements may have no effect on a person if all of the chromosome is there but just rearranged. In other cases, the rearrangements may result in pregnancy loss or birth defects.

Errors in cell division can occur soon after fertilization. This can result in mosaicism, a condition in which an individual has cells with different genetic makeups. For example, individuals with the mosaic form of Turner syndrome are missing an X chromosome in some, but not all, of their cells. Some individuals with chromosomal mosaicism may be mildly affected, but the severity of the condition depends largely on the percentage of abnormal cells.