Amniocentesis

What is amniocentesis?

This test may be performed on a pregnant woman to find out whether the unborn baby has certain genetic characteristics or birth defects.

It involves withdrawing fluid from the amniotic sac in which the foetus floats. This fluid contains waste materials and cells shed by the foetus, which can be analysed to reveal several things about the baby's genetic make-up.

While amniocentesis is not necessary for every pregnancy, it is often recommended for women over age 35, if there is a history of miscarriages (lost pregnancies), or if there is a family history of certain genetic diseases, chromosome defects, or mental retardation. It may also be done towards the end of pregnancy to determine whether the foetal lungs are mature enough to allow the baby to be delivered early, should this be advised for the health of mother or baby.

In addition to showing the presence of some abnormalities, amniocentesis can also identify the sex of the foetus, which is important if one or both parents are carriers of a sex-linked disease such as hemophilia.

It can rule out certain conditions like Down's Syndrome (a chromosomal abnormality causing mental handicap) and Tay-Sachs disease (a serious inherited metabolic disorder). It cannot detect all birth defects, but it helps to discover spina bifida (open spine), and other neural tube disorders. In all, about 80 of the possible 400 anomalies of the foetus can now be detected by amniocentesis.

What happens during amniocentesis?

You lie on a table with your hands folded on the upper chest to avoid touching the sterile area. The abdominal area is carefully cleansed, then an anesthetic is injected where the needle will be inserted to withdraw the amniotic fluid. The procedure will not begin until the area is numb.

Ultrasound is used to find the position of the foetus. This is a painless, risk-free technique that uses sound waves and their echoes to visualize internal organs such as the uterus, and the foetus. Your skin in the area to be examined is covered with a lubricant. The transducer, a machine that looks like a microphone and sends out and receives the sound waves, is passed over your skin. A computer translates these echoes into a picture on a television screen. Ultrasound is not harmful to the foetus.

The amniocentesis procedure itself involves inserting a sterile, hollow needle through your skin, the wall of the uterus, and into the amniotic sac. The doctor watches the ultra­sound monitor to make sure the needle does not puncture the baby. The needle suctions off a small quantity of amniotic fluid and is then withdrawn. A small bandage is placed on the skin. The sample of fluid is sent to the laboratory where it may be grown in tissue culture and analysed.

Does amniocentesis cause discomfort?

You will not feel anything during the test because of the anesthetic. Afterwards you may feel faint or nauseous, or perspire profusely. These aftereffects can usually be relieved by turning to lie on your left side.

How long does amniocentesis take?

The total time taken for all the procedures described above is about half an hour. The actual amount of time taken to insert and withdraw the needle is only a few minutes.

You will not be able to leave for at least an hour after the procedure is completed, so your breathing rate, blood pressure, and pulse, as well as the foe­tus' heart rate, can be monitored for any changes that may signal problems.

When will I get the results?

Some results are available almost at once, while others take a few days. Growing the cells in tissue culture for chromosomal analysis takes several weeks to complete.

Since amniocentesis cannot normally be done until the 15th week of pregnancy and all the results are not normally known for another 3-4 weeks, this precludes an abortion within the first three months should a problem like Down's syndrome be detected. An abortion can still be done if the parents so desire, but the decision is often difficult if the pregnancy is more advanced.

Is amniocentesis dangerous?

Amniocentesis is generally safe for both the mother and the foetus. But in a very small number of cases, the test precipitates a miscarriage. There is also a slight risk of infection or damage to the foetus if it is accidentally hit by the needle, but this is uncommon. The placenta may also be damaged, which can in turn cause harm to the foetus.

In addition, certain fac­tors can produce false test results, although this is rare. These include foetal blood or faeces in the test specimen or the presence of certain diseases in the mother, including certain types of cancer, viral infections like glandular fever (infectious mononucleosis), and liver disease.

Amniocentesis is not, therefore, undertaken lightly; for example, just to find the sex of the foetus. The doctor will only recommend amniocentesis if the potential benefits from the information to be obtained outweigh the risks of the procedure.

WARNING

After the test, notify your doctor if you feel:

  • Abdominal pain, cramping, or contractions.
  • Chills and fever.
  • Bleeding or fluid loss from the vagina.
  • Unusual activity or lethargy of the foetus.
tony
 

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