Amniocentesis

Removal of a sample of amniotic fluid, which surrounds a foetus in the uterus, to test for genetic or other foetal disorders. The cells that have been shed from the foetus into the fluid can be separated, cultured, and have their chromosomes examined to see whether there are any chromosomal or genetic abnormalities. Some inherited disorders (such as Down syndrome, muscular dystrophy, haemophilia and various enzyme deficiencies) can usually be detected by amniocentesis.

Analysis of amniotic fluid can also reveal some foetal disorders that are not inherited. Analysis for alphafoetoprotein can detect 9 out of 10 cases of spina bifida. The degree of foetal anaemia from rhesus isoimmunisation (an Rh incompatibility between mother and foetus) and disorders of red cell formation such as thalassaemia can also be assessed from the amniotic fluid. In premature labour, amniotic fluid can be examined for a substance called surfactant to see whether the foetal lungs are mature enough for the baby to survive after birth.

Amniocentesis is usually performed as an outpatient test at a hospital or special dinic, and takes about 15 minutes. Amniotic fluid is obtained by inserting a hollow needle through the abdominal wall and then the wall of the uterus into the amn-otic sac around the foetus. This is done under local anaesthetic with ULTRASOUND monitoring, and usually with FOETAL MONITORING. There is a little discomfort - a brief pinching pain - when the needle is inserted, and sometimes some cramping when the uterine wall is penetrated. The procedure is associated with about a 1 in 300 risk of miscarriage, only slightly greater than the risk at 15 weeks of pregnancy for women who have not had amniocentesis.

Amniocentesis is advised in circumstances where there is increased risk of inherited foetal disorders (in, for example, women over 35, those who have previously given birth to an abnormal baby, and women who have a family history of inherited birth defects), or to obtain information about other non-inherited foetal conditions. The procedure cannot be performed before 14 to 16 weeks of gesta-tion, which means that the results of genetic tests may not be available until 18 to 20 weeks. If an abnormality is discovered and termination of the pregnancy is chosen, abortion must usually be by induced labour. Amniocentesis is generally not advised for couples who are sure that they would not consider abortion if the results indicated a foetus with severe defects.

Amniocentesis is a commonly performed and generally safe procedure. Amniocentesis is sometimes used to check whether a baby is at risk of rhesus disease. The test can detect chromosomal disorders such as down's syndrome, structural defects such as spina bifida, anencephaly (a condition in which the brain is incomplete or missing), and many rare, inherited metabolic disorders. Amniocentesis can be used to diagnose a large number of genetic and chromosomal abnormalities in the fetus. In cases of family histories with chromosomal abnormalities or problematic pregnancies, some doctors will suggest younger women to take the test even though they do not fit the typical age profile. Amniocentesis is occasionally used late in pregnancy to assess whether the baby's lungs are mature enough for the baby to breathe on his own. Side-effects are the unwanted, but usually mild and temporary effects of a successful procedure. The side-effects of an amniocentesis can include some mild pain or cramps in the tummy.

When is amniocentesis offered?

Amniocentesis is usually offered:

• A woman is considered at high risk because she has previously had a pregnancy affected by a chromosomal or genetic disorder.
• If a previous test or screening result, such as the triple test for alpha-fetoprotein, estriol and human chorionic gonadotropin, or a nuchal translucency scan has suggested a high risk of a birth defect or of down's syndrome.
• Amniocentesis is occasionally required in the investigation and management of other conditions in pregnancy such as rhesus diseases.
• For women who have a history of abnormalities at birth in their family.
• For women who are over 35 years of age, as the risk of the baby having a genetic abnormality becomes greater after this age.

What are the risks and side effects to the mother or baby?

• Miscarriage is the primary risk related to amniocentesis. Miscarriages can occur because of infection in the uterus, the water breaks or labor is induced prematurely.

The mother may experience other side effects that include:

• Leakage of fluid.
• Pain or cramps in the tummy.
• Minor irritation around the puncture site.

Things to remember

• Amniocentesis is a medical procedure performed on a pregnant woman to withdraw a small amount of amniotic fluid from the sac surrounding the foetus.
• It is only performed on women thought to be at higher risk of delivering a child with a birth defect.

Precautions

• When collecting amniotic fluid universal precautions should be followed for the prevention of transmission of bloodborne pathogens.
• Some tests such as the amniotic fluid bilirubin test and alpha fetoprotein test must be interpreted with regard to gestational age.
• Parents must weigh the potential value of the knowledge gained, or indeed the reassurance that all is well, against the small risk of miscarriage.

Taking care of yourself at home

• Most women feel fine after the procedure, and no alteration to normal routine is necessary. However, it is generally advised to take it easy for the next couple of days.

Problems detected by amniocentesis

These conditions include:

• Cystic fibrosis.
• Tay-Sachs disease and sickle cell disease.
• Down's syndrome.
• Neural tube defects, such as spina bifida.

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