Amniocentesis and Chorionic Villus Sampling
Overview
Amniocentesis and chorionic villus sampling (CVS) are invasive prenatal diagnostic procedures used to obtain fetal genetic material for karyotyping, microarray, or specific gene testing. They are offered after a higher-chance result on combined/non-invasive prenatal screening, a structural anomaly on ultrasound, a known familial genetic condition, or parental carrier status. Both procedures should only be performed by operators trained to the competency standard of a subspecialty in maternal and fetal medicine or the RCOG Fetal Medicine Advanced Training Skills Module (ATSM), and always under continuous ultrasound guidance.
Gestational Timing
- CVS is usually performed between 11+0 and 13+6 weeks. If clinically required it may be extended to 14+0–14+6 weeks with individualised counselling, but should not be performed before 10+0 weeks (earlier CVS is associated with limb-reduction defects and oromandibular-limb hypogenesis).
- Amniocentesis should not be performed before 15+0 weeks. "Early amniocentesis" (before 14+0 weeks) carries a significantly higher rate of pregnancy loss (7.6% vs 5.9%; RR 1.29) and a markedly increased incidence of fetal talipes (RR 4.61) compared with later procedures, and should be avoided.
Procedure-Related Miscarriage Risk
- For singleton pregnancies, the additional (procedure-related) risk of miscarriage following amniocentesis or CVS performed by a skilled operator is likely to be below 0.5%.
- For twin pregnancies, the additional risk of miscarriage following CVS or amniocentesis performed by a skilled operator is around 1%.
- This is an additional risk above the background rate for that gestation, and risk is operator- and centre-dependent — women should ideally be counselled using local/operator-specific data where available.
Other Complications
- Amnionitis/infection is a rare but serious maternal complication, occurring in fewer than 1 in 1000 procedures.
- Transient amniotic fluid leakage or vaginal spotting can occur post-procedure and is usually self-limiting.
- Feto-maternal haemorrhage can occur with either procedure, so RhD status must be established/available for every woman undergoing the procedure.
- CVS carries a small risk of confined placental mosaicism, which can complicate result interpretation and may prompt a follow-up amniocentesis.
Anti-D Prophylaxis
Anti-D immunoglobulin must be offered to all RhD-negative, non-sensitised women following amniocentesis or CVS, in line with national anti-D prophylaxis recommendations (see GTG 22), because of the risk of feto-maternal haemorrhage and alloimmunisation.
Counselling and Consent
Detailed, individualised pre-procedure counselling should be provided by an appropriately trained professional, covering indication, procedure-specific miscarriage risk, alternative/non-invasive options, turnaround time, and the possibility of an inconclusive or unexpected result (e.g. variant of uncertain significance, incidental finding). Screening for blood-borne viruses should inform an individualised discussion of transmission risk where relevant.
High-Yield Exam Points
- CVS window: 11+0–13+6 weeks (extendable to 14+6 weeks); never before 10+0 weeks.
- Amniocentesis: not before 15+0 weeks; early amniocentesis (<14 weeks) increases pregnancy loss and talipes risk.
- Additional miscarriage risk: <0.5% singleton, ~1% twin pregnancy, when performed by a skilled operator.
- Amnionitis risk: <1 in 1000 procedures.
- Anti-D must be given to all RhD-negative, non-sensitised women after either procedure.
- CVS-specific pitfall: confined placental mosaicism may require confirmatory amniocentesis.
Source: RCOG Green-top Guideline No. 8 (25 October 2021 (reviewed December 2024; validity extended, no substantive changes))
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