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Pregnancy and amniocentesis

What is amniocentesis?

Amniocentesis is an antenatal test to check the growing baby for conditions such as Down's syndrome, spina bifida and sickle cell anaemia.

Amniocentesis may be offered between weeks 15 and 20 of pregnancy when there is thought to be a higher risk of the baby developing serious medical conditions or abnormalities.

A needle is used to take a sample of amniotic fluid from around the developing baby in the womb. This is analysed in a laboratory.

The test is relatively common, and usually safe. However, there is a small risk of complications from the procedure.

Why is an amniocentesis performed?

Because amniocentesis presents a small risk for both mother and baby, the test is generally offered to women who have a significant risk of genetic diseases, including those who:

  • Have an abnormal ultrasound.
  • Have a family history of certain birth defects.
  • Have previously had a child or pregnancy with a birth defect.
  • Will be 35 or older at the time of delivery.

Amniocentesis does not detect all birth defects, but it can be used to detect the following conditions if the parents have a significant genetic risk:

Amniocentesis can also help detect certain neural tube defects (diseases where the brain and spinal column don't develop properly) such as spina bifida and anencephaly.

Because ultrasound is performed at the time of amniocentesis, it may detect birth defects that are not detected by amniocentesis (such as cleft palate, cleft lip, club foot or heart defects). There are some birth defects, however, that will not be detected by either amniocentesis or ultrasound.

If you are having an amniocentesis, you may wish to find out the baby's sex; amniocentesis is the most accurate way to determine the baby's gender before birth.

An amniocentesis can also be done during the third trimester of the pregnancy to determine if baby's lungs are mature enough for delivery, in cases where an early delivery may be warranted. Another indication could be to evaluate for infection in the amniotic fluid in certain situations.

When is amniocentesis performed?

If your doctor has recommended an amniocentesis, the procedure is usually scheduled between the 15th and 20th week of pregnancy.

How accurate is amniocentesis?

The accuracy of amniocentesis is greater than 99%.

Amniocentesis may occasionally be unsuccessful due to technical problems such as being unable to collect an adequate amount of amniotic fluid or failure of the collected cells to grow when cultured.

Does amniocentesis have risks?

There is a small risk that an amniocentesis could cause a miscarriage (around 1%, or approximately 1 in 100). Injury to baby or mother, infection, and preterm labour are other potential complications that can occur, but are extremely rare.

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