Amniocentesis

Amniocentesis is the sampling of amniotic fluid during pregnancy. It can be done:

• Early in pregnancy (early amniocentesis between 12 and14 weeks gestation).
• Later in pregnancy (midtrimester amniocentesis between 16 and 18 weeks gestation).

The fluid extracted contains cells from the amnion and fetal skin, lungs and urinary tract. The cells are grown in culture media allowing chromosomal, genetic, biochemical and molecular biological analysis:

• It is used to achieve prenatal diagnosis. Examples of diagnosis include:
  • For management of rhesus disease or estimation of maturity.
  • Chromosomal, genetic, biochemical and molecular biological analysis of cultured amniotic cells.
• It is an invasive test posing risk to fetus and mother.
• It is not used as a screening test.^1,2

Guidance from NICE gives some perspective to the place of amniocentesis in antenatal care.³

• Advanced maternal age (>35 years) - the most common indication
• Previous child with:
  • Neural tube defect (1 in 20 subsequent pregnancies affected)
  • Chromosomal abnormalities
  • A birth defect
• Positive antenatal screening tests, including for example:
  • Fetal ultrasound findings
  • Raised maternal serum alpha-fetoprotein (ultrasound now also used in neural tube defect screening)⁴
• History of:
  • Parent carrying a balanced chromosomal translocation (1 in 4–10 chance fetus affected)
  • Risk of recessively inherited metabolic disorder
  • Mother carrying 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:
  • DNA (for example fragile X, sickle cell disease, cystic fibrosis)
  • Enzymatic activity in amniocytes (for example Tay-Sachs disease)
  • Fluid biochemistry (for example in congenital adrenal hyperplasia- 17-OH- progesterone)

• Technique:
  • Rhesus immunoprophylaxis should be given where appropriate (fetomaternal transfusion is a risk in amniocentesis and chorionic villus sampling)
  • Preferably under ultrasound guidance
  • 22G spinal needle is inserted through maternal abdominal and uterine walls into pocket of amniotic fluid within the amniotic sac
  • 10–20ml of fluid is aspirated (or approximately 1ml per week of gestation)
  • A cell filtration system may be used
  • Smaller volumes may be aspirated where advanced laboratory techniques require less material
• Midtrimester amiocentesis:
  • Normally performed in second trimester from 14–16 weeks gestation
  • There is relatively more amniotic fluid (enough amniotic fluid for reliable cell culture- about 20mls))
  • There is still time to terminate the pregnancy (if results indicate this to be advisable)
• Early amniocentesis:
  • This has been conducted at weeks 9–14
  • Less fluid is removed and ultrasound guidance is essential
  • Carries higher risk of loss of pregnancy (around 7%) and talipes equinovarus^5,6,7
  • Preferred over chorionic villus sampling (CVS) where CVS unreliable (in twin pregnancies)

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 but lecithin to sphingomyelin ratio for example)
  • Enzyme analysis (many and varied including for inborn errors of metabolism)
• On fetal cells extracted from amniotic fluid testing for genetic and chromosomal disorders:
  • Direct metaphase visualisation of chromosomes (for example for Prader-Willi syndrome)
  • Direct DNA analysis techniques (for example for Tay-Sachs disease, phenylketonuria, Duchenne muscular dystrophy and cystic fibrosis)
  • Indirect DNA analysis (used for example to detect linkage disorders when the exact gene is not known)

• Distressing symptoms (uterine cramping)
• Uterine bleeding (about 2%)
• Amniotic fluid leakage (about 3%)⁸
• There is a risk of maternal rhesus sensitisation in susceptible pregnancies (true for CVS as well)
• Amnionitis (only about 0.1%)
• Approximately 1% increased risk of pregnancy loss compared to the background risk⁶
• Failure of cell culture from 1% up to 5% if performed under 12 weeks' gestation
• Anxiety for parents caused by delay in diagnosis (may make choices for termination of pregnancy difficult)

• Chorionic villus sampling (CVS):⁹
  • Provides diagnosis in same first trimester period as early amniocentesis
  • 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)
  • Rapid results
  • Lesser risk of pregnancy loss compared to early amniocentesis (~5%)
  • Greater risk of pregnancy loss than midtrimester amniocentesis⁶
  • More fetal defects (limb reduction, oromandibular defects) especially if performed before 10 weeks' gestation
  • However, it is more technically demanding and not always available⁶

Any benefits of earlier diagnosis with chorionic villus sampling (CVS) must be carefully balanced against the greater risk of pregnancy loss compared with second trimester amniocentesis.¹⁰ 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.¹¹

It is good practice to offer genetic counselling 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.