PatientPlus articles are written by UK doctors and are based on research evidence, UK and European Guidelines. They are designed for health professionals to use, so you may find the language more technical than the condition leaflets.
Amniocentesis is an invasive, diagnostic antenatal test. It involves taking a sample of amniotic fluid in order to examine fetal cells found in this fluid.
Because it carries a slightly increased risk of miscarriage amniocentesis is usually reserved for those women considered at higher risk of carrying a fetus with a chromosomal abnormality.
Sampling of amniotic fluid during pregnancy can be done:
- Early in pregnancy (early amniocentesis between 12 and 14 weeks of gestation).
- Later in pregnancy (midtrimester amniocentesis between 15 and 18 weeks of gestation). This is the most common use of the procedure.
The amniotic fluid extracted contains cells from the amnion and fetal skin, lungs and urinary tract. The cells are then grown in culture media to allow chromosomal, genetic, biochemical and molecular biological analysis.
It is used to achieve prenatal diagnosis and to inform management. Examples include:
- Management of rhesus disease.
- Estimation of maturity.
- Chromosomal, genetic, biochemical and molecular biological analysis of cultured amniotic cells.
Genetic counselling should ideally be offered before pregnancy where there is a family history of any condition which might be tested for by either amniocentesis or chorionic villus sampling (CVS). It is clearly important to avoid unnecessary invasive testing in pregnancy where possible. It is also important to identify specific causes of abnormality (such as anti-epileptic drugs associated with neural tube abnormalities) so that preventative measures can be taken or, if not, so that appropriate and specific antenatal testing can then be offered.
It is important to offer genetic counselling both before and after diagnostic testing. Such counselling should inform parents and families about the conditions being tested for, and the implications for possible treatments and the continuation of the pregnancy.
It is also good practice to discuss the risks of amniocentesis and help patients consider what action, if any, they might wish to take should their baby be diagnosed with an abnormality. The increased miscarriage risk and small increased maternal risk associated with the procedure means that some women who feel clear that they would not wish to terminate an affected pregnancy choose not to have the test.
Indications for amniocentesis
Guidance from the National Institute for Health and Care Excellence (NICE) gives some perspective to the place of amniocentesis in antenatal care. The list of indications becomes shorter with good pre-conceptual genetic counselling and earlier recognition of increased risk, which often makes CVS a better option.
Most common indications for amniocentesis are:
- Advanced maternal age (>35 years) - the most common indication.
- A previous child with:
- Neural tube defect (1 in 20 subsequent pregnancies are affected).
- Chromosomal abnormalities.
- Any other congenital abnormality
- Positive antenatal screening tests, including for example:
- A history of:
- Parent carrying a balanced chromosomal translocation (1 in 4–10 chance of a fetus being affected).
- Risk of a recessively inherited metabolic disorder.
- Mother carrying an X-linked disorder (to determine fetal sex).
- Mother exposed to certain drugs or infections (which can cause fetal malformations).
- Analysis to detect specific conditions from:
Amniocentesis is also used much later in pregnancy to test for lung maturity.
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- Rhesus immunoprophylaxis should be given where appropriate (fetomaternal transfusion is a risk in amniocentesis and CVS).
- It is normally performed under continuous ultrasound guidance.
- A 22 gauge spinal needle is inserted through the maternal abdominal and uterine walls into the pocket of amniotic fluid within the amniotic sac
- 10–20 ml of fluid is aspirated (or approximately 1 ml per week of gestation).
- A cell filtration system may be used.
- Smaller volumes may be aspirated where advanced laboratory techniques require less material.
- This is most commonly performed at 15–16 weeks of gestation.
- There is relatively more amniotic fluid (enough amniotic fluid for reliable cell culture - about 20 ml).
- Termination of the pregnancy (if results indicate this to be advisable and the patient wishes it) can be carried out at 18-20 weeks.
- This has been conducted at weeks 9–14.
- Less fluid is removed and ultrasound guidance is essential.
- It carries a higher risk of loss of pregnancy (around 7%) and of talipes equinovarus.
- It may be preferred over CVS in situations where CVS is unreliable (eg, in twin pregnancies).
Diagnostic testing of amniotic fluid
It is useful to know what is offered locally as there is some variation across the country. The following tests can be performed:
- On the amniotic fluid:
- AFP and acetylcholinesterase levels (for neural tube defects).
- Bilirubin levels (for gestational assessment and to detect isoimmune haemolysis).
- Tests of lung maturity (various - for example, lecithin to sphingomyelin ratio).
- Enzyme analysis (many and varied including for inborn errors of metabolism).
- On fetal cells extracted from amniotic fluid testing for genetic and chromosomal disorders:
- Rapid testing (results in 24 to 72 hours). In most areas this will identify specifically:
- Chromosome analysis after cell culture (results take about two weeks). This will give full karyotyping but even this will not identify all chromosomal abnormalities.
- Other possible tests on fetal cells (uncommonly used in practice, as appropriate genetic counselling tends to preclude the need):
Patients should be advised of how and when results of testing will be available and this may vary according to the tests being done and the laboratory used.
Risks and complications of amniocentesis
- Discomfort (uterine cramping).
- Uterine bleeding (about 2%).
- Amniotic fluid leakage (about 3%).
- Maternal rhesus sensitisation in susceptible pregnancies (also true for CVS).
- Amnionitis (about 0.1%).
- 0.5-1% increased risk of pregnancy loss compared with the background risk.
- Failure of cell culture from 1% up to 5% if performed under 12 weeks of gestation.
- Anxiety for parents due to lateness of diagnosis (this may make decisions about termination of pregnancy very difficult).
Other diagnostic techniques
- Provides diagnosis in the same first trimester period as early amniocentesis.
- Is the technique of choice for prenatal diagnosis before 12 weeks of, for example:
- Chromosomal abnormalities.
- Genetic disorders (DNA diagnosis).
- Enzymatic defects (for example, congenital adrenal hyperplasia, lysosomal enzyme defects).
- Results are rapid.
- There is a lesser risk of pregnancy loss compared with early amniocentesis (~5%).
- Has greater risk of pregnancy loss than midtrimester amniocentesis.
- More fetal defects (limb reduction, oromandibular defects) especially if performed before 10 weeks of gestation.
- However, it is more technically demanding and not always available.
See separate article Chorionic Villus Sampling.
Any benefits of earlier diagnosis with CVS must be carefully balanced against the greater risk of pregnancy loss compared with second-trimester amniocentesis. There appears in some series to be no significant difference in long-term health outcomes between children who had transcervical CVS or amniocentesis for prenatal testing.
Further reading & references
- Amniocentesis and Chorionic Villus Sampling; Royal College of Obstretricians and Gynaecologists (June 2010)
- Spencer K, Spencer CE, Power M, et al; Screening for chromosomal abnormalities in the first trimester using ultrasound and maternal serum biochemistry in a one-stop clinic: a review of three years prospective experience. BJOG. 2003 Mar;110(3):281-6.
- Spencer K, Nicolaides KH; Screening for trisomy 21 in twins using first trimester ultrasound and maternal serum biochemistry in a one-stop clinic: a review of three years experience. BJOG. 2003 Mar;110(3):276-80.
- Ndumbe FM, Navti O, Chilaka VN, et al; Prenatal diagnosis in the first trimester of pregnancy. Obstet Gynecol Surv. 2008 May;63(5):317-28.
- Antenatal care for uncomplicated pregnancies; NICE Clinical Guideline (March 2008)
- Wald NJ, Rodeck C, Hackshaw AK, et al; SURUSS in perspective. Semin Perinatol. 2005 Aug;29(4):225-35.
- Alfirevic Z, Sundberg K, Brigham S; Amniocentesis and chorionic villus sampling for prenatal diagnosis. Cochrane Database Syst Rev. 2003;(3):CD003252.
- Philip J, Silver RK, Wilson RD, et al; Late first-trimester invasive prenatal diagnosis: results of an international randomized trial. Obstet Gynecol. 2004 Jun;103(6):1164-73.
- Centini G, Rosignoli L, Kenanidis A, et al; A report of early (13 + 0 to 14 + 6 weeks) and mid-trimester amniocenteses: 10 years' experience. J Matern Fetal Neonatal Med. 2003 Aug;14(2):113-7.
- Alfirevic Z, von Dadelszen P; Instruments for chorionic villus sampling for prenatal diagnosis. Cochrane Database Syst Rev. 2003;(1):CD000114.
- Evans MI, Andriole S; Chorionic villus sampling and amniocentesis in 2008. Curr Opin Obstet Gynecol. 2008 Apr;20(2):164-8.
- Alfirevic Z, Gosden CM, Neilson JP; Chorion villus sampling versus amniocentesis for prenatal diagnosis. Cochrane Database Syst Rev. 2000;(2):CD000055.
- Schaap AH, van der Pol HG, Boer K, et al; Long-term follow-up of infants after transcervical chorionic villus sampling and after amniocentesis to compare congenital abnormalities and health status. Prenat Diagn. 2002 Jul;22(7):598-604.
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Dr Richard Draper
Dr Mary Lowth
Dr John Cox