Genetic material is organized into structures called chromosomes. Most babies have 46 chromosomes that come in 23 pairs – half are inherited from the mother and half from the father.
The first 22 pairs of chromosomes are referred to as autosomes. The last pair of chromosomes are referred to as the sex chromosomes and will differ in males and females. Females typically have two copies of a chromosome called the X chromosome (XX), while males typically have an X chromosome and a Y chromosome (XY).
In rare cases, a baby will have an extra or missing chromosome or part of a chromosome. An extra chromosome is referred to as a trisomy. A missing chromosome is referred to as a monosomy. This usually happens randomly at the time of conception. Missing or extra chromosomal material can lead to pregnancy loss or genetic conditions associated with physical or intellectual disabilities, such as Down syndrome.