About 95 percent of the time, Down syndrome is caused by trisomy 21 — the person has three copies of chromosome 21, instead of the usual two copies, in all cells. This is caused by abnormal cell division during the development of the sperm cell or the egg cell.

Aneuploidy is caused by nondisjunction, which occurs when pairs of homologous chromosomes or sister chromatids fail to separate during meiosis. The loss of a single chromosome from a diploid genome is called monosomy (2n-1), while the gain of one chromosome is called trisomy (2n+1).

Nondisjunction causes errors in chromosome number, such as trisomy 21 (Down syndrome) and monosomy X (Turner syndrome). It is also a common cause of early spontaneous abortions.

Down’s syndrome is always caused by the presence of extra chromosome 21 material in a person’s cells. There are 3 types of Down’s syndrome. Most people with the condition have regular trisomy 21. Much smaller numbers have translocation, or mosaic Down’s syndrome.

Height and weight — Babies with Down syndrome are usually smaller than other babies, and they have smaller heads. They may also grow more slowly and may never reach the same heights that typical children do.

Down syndrome can’t be prevented, but parents can take steps that may reduce the risk. The older the mother, the higher the risk of having a baby with Down syndrome. Women can reduce the risk of Down syndrome by giving birth before age 35.

A–Z