Chromosomal Abnormalities

Chromosomes carry the hereditary factors or genes and are present in every cell in our body. Normally, each cell has 46 chromosomes in 23 matching pairs. One of each pair come from the mother in the egg and one of each pair from the father in the sperm. The sex of the unborn child is determined by the chromosomes – the female has 2 X chromosomes and the male an X and a Y chromosome.

Normal Chromosomes in a female fetus:


Occasionally the egg or sperm may be abnormal with less or more genetic material than normal. Imbalance may involve the loss or gain of a whole chromosome or part of a chromosome. An incorrect amount of genetic material carried by the fetus disturbs and distorts its normal growth pattern. The most common error is Down syndrome which has an extra copy of chromosome 21. The baby will have three copies of this chromosome instead of two copies resulting in a total of 47 chromosomes.
In the pregnant mother factors which can cause concern are:

• Advanced maternal age over 35 years
• Parents already known to carry chromosomal abnormalities
• Previous birth of a child with chromosomal abnormalities
• Increased risk determined by Nuchal Translucency Screening test 
• Abnormality found on ultrasound scan.

Chromosomes showing a Trisomy 21 male fetus

Approximately 1 in 5 pregnancies are chromosomally abnormal and the vast majority result in miscarriage in the first half of pregnancy. The incidence of babies born alive with a chromosomal abnormality is 1 in 200 but not all of these abnormalities result in retardation or deformity.

If your pregnancy has indicated a reason for concern you will be offered prenatal Diagnostic Invasive testing. This is done through either a chorionic villus sampling (CVS) or amniocentesis. A test that results in normal chromosomes does not guarantee a normal baby. Some genetic diseases such as cystic fibrosis cannot be detected by chromosomal analysis and specialised DNA testing is required to exclude these disorders.