Trisomy 21 (Down Syndrome)
Normally, all cells in the human body have 46 chromosomes, made up of 23 pairs of chromosomes. 23 chromosomes (one of each pair) are inherited from the father and 23 inherited from the mother. These 23 pairs of chromosomes contain all of the DNA, or building blocks, to determine the health of the baby. If, at the time of conception, one extra chromosome is passed on, the baby will have a trisomy (one extra of a particular chromosome). If that extra chromosome is a number 21 chromosome, the baby will have Trisomy 21, more commonly called Down syndrome.
In the vast majority of cases (over 95%) the chances of passing on an extra chromosome are determined by the mother's age. The older a woman is when she gets pregnant, the higher the chance that an extra chromosome will be passed on. It is important to realise that any woman can have a baby with Down syndrome, regardless of her age. For example, a 20 year old woman has about a 1 in 1,000 risk of passing on an extra chromosome causing Down syndrome, while a 40 year old woman has about a 1 in 70 chance of this happening. For most parents, the chances of having a baby with Down syndrome have nothing to do with family history or how healthy other pregnancies were.
Down syndrome is probably the most common cause of mental retardation. Babies with Down syndrome tend to develop more slowly than other babies do. For example, they may start walking later than other babies. About half are born with heart defects, or with a blockage in their intestines that prevents them from digesting food properly. These heart and intestine problems are usually fixed by surgery. Most children with Down syndrome go to regular schools, but special additional resources are usually made available as they grow and develop.
Trisomy 18 (Edwards Syndrome)
Like Down syndrome, this condition occurs when there is one extra chromosome, but in this case it is an extra chromosome number 18. Trisomy 18 is often also called Edwards syndrome. In the vast majority of cases, the chances of having a baby with Edwards syndrome are also related to the mother's age. However, Edwards syndrome is much rarer than Down syndrome. For example, a 20 year old woman has close to a 1 in 2,000 risk of passing on an extra chromosome number 18, and a 40 year old woman has close to a 1 in 150 chance of this happening.
Another big difference between Edwards syndrome and Down syndrome is that Edwards syndrome is usually considered to be lethal, with very little possibility of the baby surviving. This is because most babies with Edwards syndrome have severe mental retardation, serious heart defects, and problems with development of the brain, spine, kidneys, or intestines. Most fetuses with Edwards syndrome are very small and don't grow very well in the womb. About two-thirds of all fetuses with Edwards syndrome die in the womb before birth. A small number of babies born alive with Edwards syndrome may even survive for a few months, but this is quite rare.
Trisomy 13 (Patau Syndrome)
Trisomy 13 is quite rare, occurring in about 1 in 5,000 births. Like Down syndrome and Edward syndrome, it is caused by one extra chromosome, this time a chromosome number 13. Also called Patau syndrome, the chances of it happening are determined by the mother's age in most cases.
Like Edwards syndrome, Patau syndrome is usually considered to be lethal, with virtually no possibility of the baby surviving. Almost all babies with Patau syndrome have severe abnormalities, like serious heart defects, and problems with development of the brain, face, kidneys, limbs or intestines. Rare cases of Patau syndrome may survive for a few months after birth.