What is Down's syndrome (trisomy 21)?
Down's syndrome is caused by the presence of an extra chromosome number 21 in the cells of the developing baby. Usually it is not inherited and so a baby can be affected even if there is no history of Down's syndrome in the family.
Down's syndrome is the most common cause of severe learning disability in children. In the absence of antenatal screening, about 1 in 500 babies born would be affected.
People with Down's syndrome have varying degrees of learning disability, but most often the disability is severe. Some people will lead semi-independent lives while others will be completely dependent. About 50% of Down's syndrome pregnancies will miscarry between conception and term, but nine out of ten babies who reach term will survive their first year. About 40% of babies with Down's syndrome are born with a serious heart defect. The average life expectancy of a person with Down's syndrome is about 60 years, although most will develop pathological changes in the brain associated with Alzheimer's disease after the age of 40.
What are open neural tube defects?
Neural tube defects are one of the most common serious congenital malformations. In the absence of antenatal screening, about 1 in every 650 babies born would be affected.
The two main kinds of open neural tube defects (NTDs) are spina bifida and anencephaly. Babies born with spina bifida have an opening in the bones of the spine which can result in damage to the nerves controlling the lower part of the body. This causes weakness and paralysis of the legs and sometimes bowel and bladder problems. Babies born with spina bifida are also more likely to have a collection of fluid on the brain, called hydrocephalus, which can be treated surgically but may lead to mental handicap.
Babies with anencephaly have a large part of the skull missing and the brain is not properly formed. They always die before or soon after they are born.
In about 1 in every 5 babies with spina bifida the spinal opening is covered with skin or thick tissue. This is called closed spina bifida and will not be detected by the blood test. This condition is usually less severe than open spina bifida.
What is Edwards' syndrome (trisomy 18)?
Edwards' syndrome is a rare and usually fatal abnormality which is caused by the presence of an extra chromosome number 18 in the cells of the developing baby. In the absence of screening about 1 in every 7000 babies are born with Edwards' syndrome. The screening tests will identify pregnancies at a high risk of Edwards' syndrome.