Prenatal diagnosis
Prenatal diagnosis is the diagnosis of disease or condition in a fetus or embryo before it is born. The aim is to detect birth defects such as neural tube problems, chromosomal abnormalities and other conditions to prepare the parents for the birth.
There are both invasive and non-invasive methods of diagnosing a problem before birth. Examination the mother's belly and ultrasonography are routinely done to check for any abnormality in the pregnancy.
If an abnormality is detected by a non-invasive procedure, a more invasive technique may need to be used. Invasive techniques include amniocentesis, which can be done from about 14 weeks gestation, and usually up to about 20 weeks, and chorionic villus sampling which can be done earlier (between 9.5 and 12.5 weeks) but is slightly more risky to the unborn child.
Reasons for prenatal diagnosis
Knowing a fetus is abnormal before birth allows parents to plan for any health needs of their baby in advance of it being born. It can also prepare them for the birth, enabling them to receive counselling before birth, reducing the shock and other reactions when the baby is born. As well as this, it gives the option of abortion of the fetus.
Healthcare staff can also prepare for the delivery, and have suitable treatment ready for when the baby is born.
The type of prenatal diagnosis done depends on the situation of the parents. In an older mother (for instance over 35), or a parent with a inheritable genetic condition, a more invasive technique may need to be done. This can detect chromosomal abnormalities (such as Down's syndrome) which are more common in older mothers, or the specific genetic problem a parent may have.
Ethical and practical issues
Will the result of the test affect treatment of the fetus?
In some genetic conditions, for instance cystic fibrosis, an abnormality can only be detected if DNA is obtained from the baby. Usually an invasive method is needed to do this.
If a genetic disease is detected, there is often no treatment that can help the fetus until it is born. It does give parents the option to consider abortion of the baby.
If abortion isn't an option for a particular couple (because of their own beliefs), invasive prenatal diagnosis of such a condition is unhelpful as the test puts the child at risk, and knowing the result doesn't help the child.
False positives and false negatives
Ultrasound of an unborn baby can often miss subtle abnormalities. Similarly a fetus may look to be abnormal by ultrasound, but may be born completely healthy.
Genetic testing, while it has fewer false positives, will often not detect every form of the disease.
Both false positives and false negatives will have a large impact to a couple when they are told the result, or when the child is born.
Methods of prenatal diagnosis
Relatively non-invasive methods: (to the baby)
- Examination of the mother's uterus from outside the body. (i.e. Feeling the mother's 'stomach'.)
- Ultrasound detection - Commonly used to check the baby's sex, to look for twins, and also to check for any abnormal development.
- Levels of alpha fetoprotein, β-hCG, and estriol in the mother's serum.
- Detection of fetal blood cells in maternal blood. With this technique it is technically possible to obtain a sample of the baby's DNA using blood cells from the fetus, that have made their way into the mother's bloodstream.
More invasive methods:
- Chorionic villus sampling - Involves getting a sample of the chorionic villus and testing it. This can be done earlier than amniocentesis, but is a more risky procedure.
- Amniocentesis - This can be done once enough amniotic fluid has developed to sample. Cells from the baby will be floating in this fluid, and can be separated and tested.
- Embroscopy and
