Amniocentesis is a procedure in which a small amount of amniotic fluid is drawn off from the amniotic sac in the uterus, without harming the fetus. The fluid is then chemically tested to check on the health of your unborn baby.

Because the amniotic fluid is swallowed by the fetus and passes out through its mouth and bladder, it contains cells of the baby’s skin and other organs, which when analyzed under a microscope can give clues to the baby’s condition and its sex.

The chromosomes in the cells can be analyzed to give information about 75 different genetic disorders. Not all the tests are done routinely, so it is important for your doctor to authorize all the tests that could apply to you because of your medical and family history. There is a new technique called chorion villus biopsy which also diagnoses genetic disorders, but it is still in the experimental stage.

Why is amniocentesis done?

The major reason for doing this test is to check for chromosomal abnormalities. These are more likely to occur if:

  • The mother-to-be is over 37. Such women are more at risk of delivering a Down’s syndrome baby and may be offered this test routinely at between 16 and 18 weeks of pregnancy. Maternal age is an important factor in Down’s syndrome; the incidence rises sharply after 35.
  • Either parent has a known chromosomal abnormality. For example, if a woman is a carrier of a genetic disorder such as hemophilia, her sons will have a 50% chance of being affected. Amniocentesis reveals the sex of the baby, so if she is carrying a boy, she will need to decide whether to terminate her pregnancy.
  • A chromosomally abnormal child or one with another congenital disability has been born previously.
  • Certain diseases which run in either parent’s family including muscular dystrophy and certain errors of metabolism such as phenylketonuria.
  • The mother has suffered three or more miscarriages.
  • The baby is to be induced prematurely or delivered early by Caesarean section. The test is done late in pregnancy to determine lung maturity. Premature babies are prone to respiratory distress syndrome.
  • Neural tube defects such as anencephaly are suspected. It can check also for spina bifida if alpha-fetoprotein levels are raised.
  • There is a Rhesus incompatibility. The test will indicate whether the baby needs an intrauterine blood transfusion or careful attention after birth.

Extracting the fluid

After the possible application of a local anesthetic to your stomach, a hollow needle will be inserted through your abdominal wall into the uterus. About 14g (1/2oz) will be removed in the syringe. This will be spun in a centrifuge to separate the cells which are then cultured from 2 1/2 to 5 weeks. null

What are the risks of amniocentesis?

The risk of fatal injury is practically nil and those of spontaneous miscarriage are slim approximately one out of 200 – with trained staff and an ultrasound scan. Other complications such as infection and bleeding are also rare. Some fetal blood cells may leak into the maternal circulation, possibly resulting in complications for Rhesus negative women. The test is about 99% reliable.

The procedure, however, should not be undertaken lightly. You must weigh up the reasons for having it and whether you are prepared to have your pregnancy terminated if the tests give you cause for concern. Waiting for the results can be agonizing because your pregnancy is well established and termination will be the same as induced labor.

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