Amniocentesis

Amniocentesis withdraws a small volume of amniotic fluid through the abdominal wall under real time ultrasound imaging. The fluid contains foetal cells, which are cultured or analysed directly, and dissolved foetal products, which are measured biochemically.

It answers questions about foetal chromosomal and genetic status, infection, and open neural tube defects. Because the sample comes from the fetus, it has higher diagnostic accuracy than maternal screening, but it is an invasive test with a small risk of pregnancy loss.

Timing. The procedure is generally performed from about 15 weeks of gestation, when there is enough fluid for sampling and enough foetal cells for culture.

Usage. It is used after abnormal screening, when ultrasound shows an anomaly, in pregnancies with a known familial variant, and when a rapid diagnosis is needed.

Indications. Abnormal combined or cell free DNA screening result, ultrasound anomaly, known parental carrier status for a chromosomal rearrangement or single gene condition, maternal anxiety after a risk assessment, advanced maternal age where the individual risk is considered high, suspected maternal infection (for example, cytomegalovirus or toxoplasmosis PCR), and measurement of alpha fetoprotein or acetylcholinesterase in suspected neural tube defects.

Preparation and technique. Placental position and foetal position are mapped by ultrasound, and the needle path is selected to avoid the placenta and foetal parts. Approximately 15 to 20 mL of fluid is drawn. The first 1 mL is usually discarded, since it is more likely to be contaminated with maternal blood. Informed consent covers the risk of loss and the possibility of an incomplete or unreadable result.

Contraindications and precautions. No absolute contraindication exists. Relative cautions include active infection at the puncture site, placental position in the needle path, and uncontrolled maternal bleeding. Rh negative women are offered anti-D immunoglobulin after the procedure.

Interpretation.

• Karyotype from cultured cells usually reports in 2 to 3 weeks.
• Fluorescence in situ hybridisation (FISH) and quantitative fluorescent PCR (QF-PCR) give rapid aneuploidy results, usually within 24 to 48 hours.
• Chromosomal microarray detects copy number variants, and its report includes variants of uncertain significance that require counselling.
• Elevated amniotic fluid alpha fetoprotein, together with acetylcholinesterase positivity, suggests an open neural tube defect. Fluid levels are compared with gestational age ranges.
• PCR results for infection identify the organism, but a negative result does not exclude infection of the fetus when the maternal infection was recent.

Causes of an uninterpretable result. Culture failure, maternal cell contamination, and insufficient fluid volume.

Critical values and findings needing immediate action.

• Rapid confirmation of a serious chromosomal abnormality requires prompt counselling before the culture result.
• Confirmed infection in the fetus requires specialist foetal assessment without delay.

Limitations and artefacts. Blood contamination dilutes the alpha fetoprotein level and causes false elevations in some assays. Maternal cell contamination can give a misleading karyotype, and this is checked with a parental marker or a second sample. Mosaicism in the cultured cells may not reflect the whole fetus.

Follow up tests. Confirmatory karyotype or microarray on a second sample when mosaicism or an unexpected finding is reported, detailed foetal ultrasound, foetal echocardiography where a cardiac anomaly is present, and genetic counselling.

Complications. Pregnancy loss is the main concern and is generally quoted as a small risk, in the region of 0.1 to 0.3%, varying with operator experience and centre. Other complications include leakage of fluid, bleeding, infection and needle injury to the fetus, which is rare.

Special groups. The procedure is adjusted for multiple pregnancy, where each sac is sampled separately. In women with uterine anomalies, the needle path is planned with additional imaging.