Amniocentesis

Amniocentesis is an invasive prenatal diagnostic procedure undertaken primarily to detect genetic and chromosomal abnormalities in the developing fetus. It involves the transabdominal insertion of a fine needle into the amniotic cavity, allowing withdrawal of amniotic fluid that contains fetal cells and biochemical constituents. The technique has become a cornerstone of prenatal diagnosis since the mid-twentieth century, although the concept of aspirating amniotic fluid dates back to the late nineteenth century in the management of hydramnios. Modern practice aligns with clinical guidelines that recommend offering prenatal genetic assessment to all pregnant individuals, irrespective of age or risk factors.

Procedure and Mechanism

Amniocentesis is most commonly performed between the fifteenth and twentieth gestational weeks. Ultrasound guidance is used throughout to avoid placental structures and fetal contact. Once the amnion is punctured, a small volume of amniotic fluid is aspirated for laboratory analysis. This fluid contains desquamated fetal cells, which can be karyotyped or subjected to DNA-based technologies to detect specific genetic abnormalities. Because the test is invasive, it carries a measurable risk of pregnancy loss, typically estimated at 0.1–0.3 per cent, with higher risks associated with procedures attempted before fifteen weeks.
Relative contraindications exist—such as maternal infection or coagulopathy—yet no absolute contraindications have been identified. Decisions regarding testing should follow a shared decision-making process that incorporates the individual’s clinical risks, preferences and values.

Diagnostic Indications

Genetic DiagnosisAmniocentesis is a gold-standard diagnostic test for confirming the presence of many genetic conditions after fifteen weeks of gestation. It complements non-invasive prenatal screening and is often considered when screening results are high risk or when ultrasonography identifies anomalies. Indications include advanced parental age, prior offspring with genetic disease, known parental chromosomal rearrangements, a family history of hereditary disorders or the detection of fetal structural abnormalities.
The analysis of amniotic fluid enables identification of a wide spectrum of conditions. Commonly detected chromosomal aneuploidies include Down syndrome, Edwards syndrome, Patau syndrome, Turner syndrome and Klinefelter syndrome. Microdeletion and microduplication syndromes, such as DiGeorge syndrome and Cri du chat syndrome, can also be identified. Amniocentesis facilitates molecular testing for single-gene disorders including cystic fibrosis, haemophilia, Duchenne muscular dystrophy, Tay–Sachs disease, sickle cell disease, neurofibromatosis and other familial conditions.
Although comprehensive, prenatal genetic testing cannot detect all potential abnormalities or predict phenotypic severity with complete accuracy.
Assessment of Fetal Lung MaturityHistorically, amniocentesis was used to evaluate fetal lung development in pregnancies complicated by maternal or fetal conditions that might necessitate early delivery. Surfactant concentration in the amniotic fluid—measured by tests such as the lecithin–sphingomyelin ratio, phosphatidylglycerol presence and surfactant–albumin ratio—served as a proxy for pulmonary maturity. However, contemporary guidance suggests that if early delivery is clinically required, it should proceed regardless of biochemical indications of lung maturity. Conversely, if deferral is possible based on lung immaturity, invasive testing is rarely justified. As a result, the practice of performing amniocentesis solely for this purpose has declined, except when gestational age is uncertain.
Assessment and Management of InfectionAmniocentesis may be used to diagnose intra-amniotic infection, such as chorioamnionitis, by evaluating amniotic fluid via Gram stain, culture or biochemical testing. In practice, the diagnosis is typically made clinically to enable timely management. The procedure can detect congenital infections, including cytomegalovirus, hepatitis B, parvovirus B19 and toxoplasmosis, thereby assisting in prognostication and management strategies.
Assessment of Rh IsoimmunisationRh isoimmunisation develops when an Rh-negative pregnant individual forms antibodies against fetal Rh-positive red blood cells, potentially leading to haemolytic disease of the fetus and newborn. Although serial amniocenteses were once used to monitor isoimmunised pregnancies by assessing amniotic fluid bilirubin levels, Doppler ultrasound evaluation of the fetal middle cerebral artery peak systolic velocity has largely replaced this approach. Amniocentesis thus plays a diminished role in this context, though it remains relevant in specific diagnostic scenarios.

Therapeutic Applications

Beyond diagnosis, amniocentesis may be employed therapeutically. In severe hydramnios, controlled drainage of amniotic fluid can alleviate maternal discomfort and reduce risks such as preterm labour. This therapeutic aspiration differs from diagnostic sampling in scope but utilises the same transabdominal approach.

Risks and Considerations

Amniocentesis is generally safe when performed by experienced practitioners, but its invasiveness introduces potential complications. These include pregnancy loss, infection, amniotic fluid leakage, fetal injury and Rh sensitisation in unsensitised Rh-negative individuals. Prophylactic administration of RhD immune globulin is standard for eligible patients to reduce the risk of alloimmunisation. As with any invasive procedure, clinicians must weigh risks against diagnostic value while maintaining informed patient engagement.

Historical Context and Development

The evolution of amniocentesis reflects advances in imaging, genetics and prenatal care. Early uses focused on decompression of excessive amniotic fluid, but the mid-twentieth century marked its emergence as a diagnostic tool with the establishment of chromosomal analysis techniques. Subsequent developments in molecular genetics expanded its utility, making it central to the diagnosis of hereditary and chromosomal disorders. Its role continues to evolve alongside non-invasive testing modalities, which alter the balance between screening and diagnosis but do not replace the definitive nature of invasive cytogenetic assessment.