What is Down's syndrome?
Down's syndrome (or Down syndrome) is a chromosomal disorder where the affected individual will have a number of abnormal physical and mental characteristics. The most significant characteristic is the considerable learning difficulties that the affected person has. A Down's syndrome individual rarely develops a mental age above that of a five year-old.
What are the physical characteristics of a Down's baby?
The head tends to be flat-ish, both in the front and the back. The eyes are slanted (hence the older, rather insensitive term "mongoloid", and the ears have a square-ish appearance and are rather low-set. The arms may appear slightly short and there may be abnormal creases on the palms of the hands. The tongue tends to appear rather too large for the mouth.
Apart from the external physical features, a Down's baby may be born with major internal organ abnormalities. Organs particularly affected are the heart and the gut. Some abnormalities could be life-threatening or require major corrective surgery.
What happens when a Down's baby grows up?
Apart from the learning difficulties of variable severity, physically a Down's syndrome individual tends to be of short stature. Most of them attain a maximum height of less than five feet (152 cm). Many are a lot shorter than this.
What happens to cause the baby to end up with Down's syndrome?
As mentioned earlier, this is a chromosomal disorder.
A normal individual has twenty-three pairs of chromosomes, making a total of forty-six chromosomes. Each and every cell in the body, except for gametes, has forty six chromosomes. In Down's syndrome, there are forty-seven chromosomes.. The extra chromosome is on pair number twenty-one. This means on ‘pair’ number 21 there are three instead of two chromosomes. This is why this condition is also called Trisomy 21. There are other chromosomal disorders where there is an extra chromosome, as we shall see below. Down's syndrome or Trisomy 21 is the most common chromosomal disorder compatible with life.
Are there any predisposing factors for Down’s Syndrome?
It is important to be aware that most Down's syndrome babies are born to mothers who have no known predisposing factors. It means, therefore, that without screening tests, all these will come as a complete surprise. However, there are people who are more likely to have Down's syndrome children because of a number of factors. The most important factor is maternal age.
Age of Parents and risk of Down’s syndrome
How important is maternal age?
The older the mother, the higher the risk of a Down's syndrome pregnancy. Even though a Down's baby can be born to a mother of any age, the risk increases significantly above the age of thirty-five. The risk to a mother aged twenty is close to 1 in 1500, rising to 1 in 900 at 30 years, 1 in 350 at thirty-five and as high as 1 in 85 at forty. That is more than 1% risk. The risk is an extremely high 1 in 35 by the time the mother is 45. As can be seen, the risk rises more than 30-fold at the age of forty five compared to the risk of a twenty-year old mother.
Is paternal age important?
No. There is no evidence that the father's age influences the risk of having a Down's syndrome baby in any way.
Table showing risk of having a baby with Down's syndrome by maternal age.