Updated 18 September 2015

Amniocentesis (procedure)

Procedure used to obtain a sample of amniotic fluid from the uterus of a pregnant woman.



  • Amniocentesis is the removal of amniotic fluid through a needle.
  • The needle is inserted through the pregnant woman's abdomen into the uterus and amniotic sac.
  • The test is performed in order to gain information about the foetus.
  • The aspirated fluid contains foetal cells which are examined for possible abnormalities.
  • The test is usually performed during the 14th and 18th week of pregnancy.

Alternate names

"amnio", amniocentesis test

What is an amniocentesis?

Amniocentesis is a procedure whereby a small amount of amniotic fluid from the amniotic sac is removed. The amniotic sac contains the foetus suspended in the amniotic fluid. The test can be performed at different times during pregnancy as from 14 weeks onwards. If done between the 15th and 18th week of pregnancy, the test can give important information about the chromosomes and possible developmental defects of the baby.

If performed later in pregnancy, it can measure the maturity of the baby's lungs and determine whether the baby may have problems with breathing.

Amniocentesis is a fairly common (but not routine) test.

Who needs an amniocentesis?

A doctor may recommend the test if a pregnant woman has already had a previous child with a genetic problem (such as cystic fibrosis or sickle cell disease), or if she is 35 years or older as the risk of foetal chromosomal abnormalities increases with age.

If there is an indication to deliver the baby earlier than the due date, the doctor may sometimes recommend amniocentesis to check for lung maturity.

How is an amniocentesis performed?

Before the amniocentesis, a detailed ultrasound is performed first. The foetal measurements are taken, including the head diameter and circumference, the abdominal circumference and the length of the femur (thighbone). When combined mathematically, these measurements give an estimate of gestational age (age of the baby from the time of conception) to within two weeks. The previously estimated due date will not change unless the measurements indicate a difference of at least ten days.

Next, a detailed anatomic survey of the foetus is carried out. This includes the anatomy of the head, brain, spine, chest, heart structure, stomach, gall bladder, kidneys, bladder, umbilical cord, arms and legs. The location of the placenta and the amount of amniotic fluid are assessed, and the maternal organs are examined for any uterine fibroids or ovarian cysts. The cervix will be scanned to identify any dilatation or where it is in relation to the placenta.

Finally, an amniocentesis site is chosen under ultrasound guidance to avoid any contact with the placenta and baby. This is only a preliminary site and may be changed just before insertion of the needle if the baby has moved in the interim. The skin area around the site is then cleaned with alcohol or Betadine and the ultrasound probe is placed on to the tummy again to look at the chosen path of the needle.

The doctor then inserts a needle through the abdomen and uterine wall into the fluid-filled amniotic sac. Most doctors will not use a local anaesthetic, as the anaesthetic needle usually causes more discomfort than the amniocentesis needle. A small amount of amniotic fluid (about 30 cc or two tablespoons) is withdrawn and sent for testing.

Most women find amniocentesis about as uncomfortable as having blood drawn or as experiencing mild menstrual cramps. The needle is generally in and out of the abdomen in less than two minutes, although the entire procedure may take up to an hour. This is to allow for monitoring of the baby's heart rate and for further explanations.

What tests are conducted on the amniotic fluid?

Once the fluid has been withdrawn it is sent to a laboratory for two different types of testing:
  • Chromosome analysis: The cells in the fluid consist of cells from the amniotic membrane (amniocytes) and foetal skin cells. As they are not actively growing, the cells need to be cultured and stimulated which takes between two and three weeks. After that, the cells are harvested and treated to make their chromosomes visible. The cells are then broken to release the chromosomes, which are stained so that the geneticist can see the bands on each chromosome and identify them. The chromosomes are counted and matched up in 23 pairs. They are then examined for evidence of missing or extra pieces. This test also determines the baby's sex. The most common reason for chromosome analysis is to look for evidence of trisomy (three copies of a chromosome instead of the expected two). The trisomies 13 (Patau´s syndrome), 18 (Edward's syndrome) and 21 (Down syndrome) are the most common trisomies identified. More than one in twenty foetuses have evidence of trisomy if the mother is over 45 years old.
  • Alpha-fetoprotein (AFP): This test can help determine if there is an opening in the foetal skin. The most common place for this to occur is the spine. This would suggest a neural tube defect such as spina bifida (walls of spinal canal are not properly fused) or anencephaly (absence of brain and part of the skull). A woman who is taking valproic acid or carbamazepine for a seizure disorder has an increased risk of a neural tube defect in her baby. The AFP test can also be performed as a blood test from the expecting mother, however, elevated levels in the amniotic fluid are a better predictor of possible congenital defects in the baby.

What are the advantages of amniocentesis?

Amniocentesis is a relatively safe procedure in the hands of an experienced doctor. In most cases, the obvious benefits of knowing about any abnormality of the foetus prior to birth outweigh the risks of the procedure.

Before the test, both parents should get adequate counselling to understand the significance of any potentially abnormal results.

What are the risks of amniocentesis?

Miscarriage. This happens spontaneously in 15-20% of all pregnancies, usually in early pregnancy and before the time that amniocentesis would be performed. After amniocentesis, the risk of miscarriage is increased by less than 1% for the remainder of the pregnancy.

Discomfort. Cramping may occur during the amniocentesis and for a short while afterwards. This is not significant. Bleeding and leaking of amniotic fluid occur only rarely and may infrequently be followed by miscarriage.

Infection. This is very rare. If it does happen, it usually occurs 24 to 72 hours after the procedure.

Possible development of Rhesus sensitisation. Rh-negative women will be given an injection of Anti-D (Rh) immunoglobulin just after the amniocentesis. This medication can prevent Rhesus factor complications with future pregnancies.

Repeat testing required. In less than 10% of cases, a second needle insertion is required. This does not necessarily mean there is anything wrong with the baby. Sometimes it is better to wait a week before a second attempt is made. The second attempt is almost always successful.

What happens when the results of the amniocentesis become available ?

It will take two to three weeks before the results of the test become available. Since amniocentesis is an invasive procedure, it is usually preceded with non-invasive investigations (detailed ultrasound scan, blood tests), and counselling of the expecting mother or both parents about the next step to be done.

If any foetal abnormalities have been detected, a full explanation will be given by the doctor which will include all the possibilities ranging from termination of pregnancy to neonatal surgery depending on the degree and severity of the congenital defect.

Reviewed by Prof Bert Schaetzing, MD, FCOG(SA), FRCOG, PhD, part-time consultant, Dept of Obstetrics & Gynaecology, Faculty of Health Sciences, University of Stellenbosch.


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