Amniocentesis is a transabdominal procedure in which, after cleansing the lower abdomen and painting the part with antiseptic, sterile isolation towels are placed to expose only the hypogastric region. A fine needle is introduced under ultrasound guidance into the amniotic cavity to draw off a sample of amniotic fluid. This procedure is commonly performed at about 16 weeks of gestation. During the procedure, care should be taken to avoid the placental site. This fluid is used for prenatal genetic disorders, foetal karyotyping to exclude Down syndrome and chromosomal anomalies, to test for inherited metabolic disorders, and to assess alpha-foetoprotein levels as a test for open neural tube defects.
Amniocentesis may be performed later in pregnancy for other indications like:
1. Assessment of foetal pulmonary maturity:
Lecithin I sphingomyelin (L I S) ratio: A ratio >2 indicates satisfactory pulmonary maturity. The risk of neonatal respiratory distress syndrome is <2%. A L/S of 1.5-1.9 indicates risk of respiratory distress syndrome (RDS) of 50% or more, and L/S of < 1.5 indicates a risk of RDS >75%.
Phosphatidyl glycerol (PG): Its presence virtually eliminates the risk of RDS.
It is important to realize that a compromised neonate may develop RDS even in the presence of a reassuring lung maturity profile.
Shake test or foam test: This test depends on the presence of surfactant to generate a stable foam in the presence of ethanol. This is a rapid bedside screening test to assess pulmonary maturity. It is useful and the conclusions valid in the absence of blood and/or meconium. The foam stability test is a very useful adjunct whenever the facility for US testing is not available.
2.Optical density (OD) 650: Amniotic fluid turbidity is believed to be dependent on the total phospholipid concentration. OD 650 > 0.15 correlates very well with absence of RDS.
Contamination with blood and meconium invalidates the test.
3.Assessment of severity of isoimmunised pregnancies: Spectro-photometric analysis permits the assessment of severity of isoimmunization. The results of the tests are serially plotted on Liley's chart. Clinical management options of intrauterine foetal transfusion, continuation of pregnancy to achieve better foetal maturity or induction of preterm labour/caesarean section and the preparation for neonatal exchange transfusion after delivery, are based on the available laboratory data inputs and the clinical judgement of the obstetrician and neonatologist.
Potential complications of amniocentesis
Premature rupture of membranes in 1-2% of patients.
Foetal bleeding from trauma to the foetus, placenta or umbilical cord.
Direct foetal injury.
Preterm labour/miscarriage in <1% cases.
Haemorrhage from trauma to epigastric or uterine vessels.
Isoimmunization.. Protect all Rh negative women with prophylactic injection of anti-D.
Infection risks are low if proper aseptic precautions are observed.
Higher foetal loss.