Amniocentesis is a procedure during pregnancy where a small amount of the fluid surrounding the foetus (known as amniotic fluid) is withdrawn by needle from the uterus under ultrasound guidance.

When needed, amniocentesis is usually performed around the 15th week of pregnancy. The purpose of amniocentesis is to detect whether a baby may have a chromosomal abnormality such as Down syndrome, or a problem with their brain or spinal cord such as spina bifida. The test can also detect the presence of some inherited genetic disorders for couples known to have an increased risk of these conditions.

From the amniotic fluid, a number of tests can be done such as alpha-fetoprotein, which has information about your baby’s health. It is also an accurate way to find out your baby’s sex.

The test can be recommended for:

  • Expectant mothers who have had an abnormal ultrasound, or maternal serum screening test with a 'high risk' result.
  • You have a family history of birth defects.
  • You have already had a child/pregnancy with a birth defect.
  • You are aged 35 years or over when the baby is due.

Occasionally the test may be done through special arrangements with the genetic service to detect other inherited conditions such as cystic fibrosis, haemophilia or certain types of muscular dystrophy.

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Is there any preparation needed before the test?

No preparation is needed. A little, about 28gm of, fluid is taken with a fine needle inserted into your uterus through your abdomen. An ultrasound probe is used to help 'see' during the test.

What are the risks?

Amniocentesis is a relatively safe procedure but there is a small risk of miscarriage. About 0.5% (1 in 200) of women will miscarry as a result of the procedure. 

What do I look for after amniocentesis?

There may be some discomfort and some women experience period-like cramps, but these usually resolve within 24 hours. Two paracetamol tablets can be taken to ease these cramps (they will not harm the baby).

Women are advised to rest at home with no heavy lifting or strenuous activity after the amniocentesis. If you have any bleeding or fluid leaking from your vagina, you should contact your doctor, midwife or the hospital immediately as you could be at risk of having a miscarriage.

Results – what do they mean?

The results give you time to get ready for a baby that may have special needs or make a decision about whether you let your pregnancy go full term. You can find out about surgery the baby may require before it is born, or begin to change things about the way you work and live so you can look after a child who needs extra help.

Learn more 

Amniocentesis section NHS Choices, UK

Information for healthcare providers

Technique modifications for reducing the risks from amniocentesis or chorionic villus sampling Cochrane Pregnancy and Childbirth Group, 2012

Credits: Health Navigator Editorial Team . Last reviewed: 16 Jan 2015