Section 1 - What are chromosomes and what causes chromosomal changes in babies?

Each cell in the body is usually made up of 23 pairs of chromosomes. Parents contribute one chromosome from the sperm and the egg to each pair of chromosomes in the baby.

• Sometimes changes can happen by chance in the sperm or the egg which lead to an extra chromosome in all or some of the baby’s cells. Most chromosomal conditions in babies are caused this way. Nothing you or the baby’s father has done or not done can make any difference to this type of condition
• Sometimes changes can happen by one chromosome breaking off and attaching to another chromosome. These types of changes are often referred to as translocations, and in Down’s syndrome, for example, they happen in around (4%)  (1 in 25) of cases.
• Occasionally, in around (1%) (1 in 100) of cases some of the cells in the body are affected by the changes and some are not. This tends to make the effect of the chromosomal changes less serious. This is known as a mosaic chromosomal change.

The most common chromosomal change is called Down’s syndrome. This is caused by an extra copy of chromosome 21 in some or all of the cells. An extra copy of chromosome 18 causes Edwards’ syndrome. An extra copy of chromosome 13 causes Patau’s syndrome. The picture below shows the chromosomes from the cells of a baby as seen under a microscope. There is a square around the three copies of chromosome 21 that cause Down’s syndrome − you will see that all the others are pairs.

There is a wide range of potential changes to the chromosomes that may affect babies in different ways.

If the midwife or obstetrician suspects any other chromosomal changes in your baby, they will talk to you about those conditions