Amniocentesis: The operation


Amniocentesis: The operation

If you are considering having an amniocentesis test or have one planned, it is important to know all you can about it. This includes: 

  • why you need this test
  • what it will be like
  • how it will affect you
  • what risks are involved
  • any alternatives.

 The information here is a guide to common medical practice. Each hospital and doctor will have slightly different ways of doing things, so you should follow their guidance where it is different from the information given here. Because all patients, conditions and treatments vary it cannot cover everything. Use this information when making your treatment choices with your doctors. You should mention any worries you have. Remember that you can ask for more information at any time. 

What is an amniocentesis test?

This is a test to check for abnormal chromosomes in your baby. Chromosomes are tiny structures that lie within each cell of the body. There are 46 chromosomes in a human cell. There are 23 from the mother and 23 from the father, arranged as 23 pairs. They determine whether you will be male or female, how you will look and, more importantly, how your body works. Abnormal chromosomes may cause conditions, such as Down’s syndrome. During the test we take a small sample of the amniotic fluid, which surrounds the developing baby in your uterus. The sample is sent to the laboratory for examination. The test can be done any time after 16 weeks gestation.

What is Down’s syndrome?

This is a condition caused by an extra chromosome in pair 21. It is also called Trisomy 21. People with Down’s syndrome have low intelligence, slanting eyes and a flat face. They have a single crease in the palm of their hands, called a simian crease and a widened gap between their first and second toes, called a sandal gap.

More importantly, they can have serious heart defects and abnormalities of their bowels and kidneys. They are also prone to other diseases, such as leukaemia, and mental disorders. However, people with Down’s syndrome can also have a normal life span and some children are more affected than others. 

Why am I having an amniocentesis test?

There are many reasons for doing an amniocentesis test. These include:

  • A screen positive blood test - During pregnancy you have special blood tests at 16 weeks gestation. These test for your chances of having a baby with Down’s syndrome. If the results are positive your baby has a higher chance of having Down’s syndrome. We do an amniocentesis to find out whether your child definitely has Down’s syndrome.
  • Abnormalities seen on ultrasound scan - Pregnant women have an ultrasound scan at 12 weeks gestation. This is called a nuchal translucency scan. During the scan we look at the baby in your womb and check for an increase in fluid behind the baby’s neck. If an increase is seen, this gives you a higher chance of having a baby with Down’s syndrome. We do another ultrasound scan at 20 weeks gestation. This is called an anomaly scan. This scan checks for any abnormalities and other signs of Down’s syndrome, such as heart defects and kidney problems. Following either scan, if there is any cause for concern, an amniocentesis can be done to check whether the baby has Down’s syndrome.
  • Age of the mother - All women have a risk of having a baby with Down’s syndrome but the risk increases with age. Older mothers have a higher risk so we may offer you an amniocentesis test if you are over 37 years of age.
  • Previous abnormalities - If you have had a previous pregnancy affected by abnormal chromosomes we may offer you an amniocentesis test.

If you are offered an amniocentesis test the obstetrician will counsel you about the possible outcomes. S/he will discuss the various options, including not doing a test, doing a test taking the risks into consideration and what to do in the event of a positive result, including a termination of the pregnancy.

The aim

The aim of an amniocentesis is to check your unborn baby for any abnormal chromosomes. The test is 98% accurate.

The benefits

The test will confirm whether your baby has an abnormality or not. A normal result will put you and your partner’s minds at rest. An abnormal result will tell you if the baby has Down’s syndrome. This would enable you to make a decision to continue with the pregnancy or not.

 Are there any alternatives?

We can do an amniocentesis test after 16 weeks gestation of the pregnancy. If you need a test for abnormality earlier than this we would do a chorionic villus sampling (CVS). We can do a CVS any time after 11 weeks gestation.

 What if you do nothing?

If you do not have the test you will not know if your baby has an abnormality. If you are in a high-risk group we advise having the test.

 The doctor will gently insert the needle through your tummy and into your uterus. S/he will then push the needle into the amniotic sac that surrounds the baby. The amniotic sac also contains the amniotic fluid. A syringe is connected to the needle and a small sample of the amniotic fluid, about 10-15ml, is taken out. We place the sample in the specimen bottle that you previously checked. We send it to the laboratory for testing. The doctor removes the needle and places a dressing over the needle entry site. If you need the Anti-D injection it will be given now. This injection is given into the muscle in your arm, buttock or thigh. 

How long does it take?

Including the ultrasound scan, an amniocentesis test takes between 10 and 15 minutes.

Who will be doing it?

An obstetrician will do the test. They may be a specialist in fetal medicine. There will be a consultant obstetrician in charge, but a suitably experienced obstetrician may actually do the procedure. If the consultant is satisfied with the experience of that obstetrician, s/he may not be present during the test. Your regular obstetrician may not do the procedure as some units have an obstetrician that performs all the amniocentesis tests. A sonographer may do the ultrasound scan. A sonographer is a specially trained member of the x-ray staff that does many of the ultrasound scans in the hospital.



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