Fetal medicine · Invasive testing

Chorionic villus sampling versus amniocentesis — what to know before deciding

Chorionic villus sampling (CVS) and amniocentesis are the two invasive prenatal tests used when genetic information about the fetus is needed. Each samples a different tissue at a different gestational age and carries its own small procedural risks. This page describes both, when each is appropriate, what they can and cannot test for, and how the consent conversation is structured.

When invasive testing is offered

• High-risk result on combined first-trimester screening or NIPT
• Family history of a known monogenic disorder where testing is available
• Family history of a chromosomal rearrangement
• Anomaly identified on ultrasound where chromosomal explanation is being sought
• Parental balanced translocation
• Maternal request after structured counselling

Invasive testing is never imposed. It is a choice, made after the couple understands what it does and does not tell them.

CVS — chorionic villus sampling

• Gestation window: typically 11–13+6 weeks
• Tissue: chorionic villi (placental tissue of fetal origin)
• Approach: transabdominal or transcervical, under continuous ultrasound guidance
• Procedure risk: miscarriage risk in expert hands is approximately 0.1–0.5 per cent above background
• Result time: rapid result (QF-PCR for common aneuploidies) within 2–3 days; full karyotype or microarray within 2–3 weeks
• Limitation: confined placental mosaicism may give a result that does not reflect the fetus itself

Amniocentesis

• Gestation window: typically 16 weeks onwards
• Tissue: amniotic fluid containing fetal cells
• Approach: transabdominal, under continuous ultrasound guidance
• Procedure risk: miscarriage risk in expert hands is approximately 0.1–0.3 per cent above background
• Result time: rapid result (QF-PCR) within 2–3 days; full karyotype within 2–3 weeks; microarray within 2–4 weeks
• Advantage: directly samples fetal cells; no confined placental mosaicism issue

Choosing between the two

• CVS is earlier and gives earlier results — useful when timing matters (later termination is more complex)
• Amniocentesis is slightly lower-risk in some series and avoids the confined placental mosaicism issue
• If the indication arises after 14 weeks, amniocentesis is usually the only option
• Couples sometimes prefer earlier diagnosis; others prefer the lower-risk procedure
• The conversation respects the couple’s values

What the tests can detect

• Common aneuploidies (trisomies 13, 18, 21; sex chromosome anomalies) — via QF-PCR or FISH within days
• Full karyotype (numerical and major structural chromosomal abnormalities)
• Chromosomal microarray (sub-microscopic deletions and duplications)
• Specific monogenic disorders if a known parental mutation is being tested
• Whole exome sequencing in selected research/clinical contexts

What the tests cannot detect

• Many structural anomalies — ultrasound is the imaging tool for these
• Polygenic conditions (most birth defects in healthy-couple pregnancies)
• Late-onset adult disease risk
• Mosaicism that is below the threshold of the technique used
• Functional or behavioural outcomes

The consent conversation

Consent is taken without time pressure. The conversation covers: the indication, what the test will and will not tell, the procedural risk, the time to results, what the couple would do with a result (this matters because it shapes the test choice), the alternative of accepting screening risk without invasive confirmation, and the option of further imaging if anomaly is the concern.

Aftercare

• Light activity restriction for 24 hours post-procedure
• Anti-D prophylaxis where indicated
• Awareness of pain, bleeding, or amniotic fluid leak as reasons to call
• Follow-up appointment to discuss results
• Bereavement and counselling support if results indicate a difficult decision

When to refer

• High-risk combined first-trimester screening result
• High-risk NIPT result — with discussion that confirmation by CVS or amniocentesis remains standard before any major decision
• Anomaly identified on routine scan
• Family history requiring fetal genetic information

Guidelines we follow on this topic

• ISUOG Practice Guideline on Invasive Procedures
• RCOG Green-top on Amniocentesis and CVS
• SMFM Consult Series on Invasive Testing

Related reading

• Fetal medicine programme
• Fetal medicine scans
• Fetal medicine conditions
• Pregnancy care
• High-risk pregnancy
• Menstrual cycle physiology

Dr. Priyadatt Patel

Senior Gynecologist · Advanced Laparoscopic Surgeon · IVF and Endometriosis Programme Lead

MS OBGyn · Pregnancy Care · Advanced Gynaecological Ultrasound · Fertility Preservation

ESHRE / ESGE / AAGL / ASRM guideline-aligned practice. 3D Karl Storz precision technique. Fertility-preservation-first philosophy. Evidence-based decisions, honest counselling, long-term outcomes orientation.