Prenatal Care & Testing
Amniocentesis vs. CVS: Indications, Risks and Results
A side-by-side guide to the two diagnostic prenatal tests that provide definitive chromosomal answers — who needs them, how they differ, and what the real miscarriage numbers say.
Clinically reviewed · June 2026
Amniocentesis and CVS are the only prenatal tests that deliver a definitive chromosomal diagnosis. CVS is done at 10–13 weeks; amniocentesis at 15–20 weeks. In experienced hands, ACOG places the procedure-related miscarriage risk at roughly 1 in 769 for amniocentesis and 1 in 455 for CVS — meaningful but lower than many families expect.
When a screening result — whether from NIPT, a serum screen, or an anomaly spotted on ultrasound — raises a concern about the fetus's chromosomes, the next step in clinical care is often a conversation about diagnostic testing. Screening tests estimate probability; diagnostic tests answer the question definitively. Chorionic villus sampling (CVS) and amniocentesis are the two available diagnostic options, and understanding how they differ can help you and your care team make the decision that fits your timeline, your risk tolerance, and your family's values.
This article provides general educational information, not individualized medical advice. Always discuss your specific circumstances, gestational age, and lab options with your OB-GYN, maternal-fetal medicine specialist, or genetic counselor.
How do amniocentesis and CVS actually work?
Both procedures retrieve fetal genetic material under continuous ultrasound guidance — the real-time imaging is what makes them safe enough to be offered routinely in experienced centers. What they retrieve, and when, is where they diverge.
Chorionic villus sampling (CVS) removes a small sample of chorionic villi — the finger-like projections of placental tissue that are genetically identical to the fetus. The sample can be retrieved by one of two routes: transabdominal (a needle through the mother's abdomen) or transcervical (a thin catheter passed through the cervix). The operator chooses the route based on placental location and anatomy. CVS is performed between 10 and 13 weeks of gestation, making it the earliest available option for a definitive chromosomal answer.
Amniocentesis withdraws a small volume — typically 20 to 30 mL — of amniotic fluid from the sac surrounding the fetus. That fluid contains cells shed by the fetus (amniocytes), which carry the fetus's full chromosomal complement. The procedure is performed transabdominally under ultrasound guidance, most commonly between 15 and 20 weeks of gestation. In some clinical situations (such as testing for fetal infection or assessing lung maturity in late pregnancy), amniocentesis may be performed later in the third trimester.
| Feature | CVS | Amniocentesis |
|---|---|---|
| Gestational timing | 10–13 weeks | 15–20 weeks (sometimes later) |
| What is sampled | Placental villi (tissue) | Amniotic fluid (fetal cells) |
| Route | Transabdominal or transcervical | Transabdominal |
| ACOG miscarriage risk | ~1 in 455 (0.22%) | ~1 in 769 (0.13%) |
| Confined placental mosaicism | ~2% of results (may need follow-up amnio) | Not applicable (samples fetal cells directly) |
| Result turnaround (CMA) | 5–10 business days | 5–10 business days |
| Rapid FISH available | Yes (24–48 hours) | Yes (24–48 hours) |
| Primary analytic method (ACOG) | Chromosomal microarray (CMA) | Chromosomal microarray (CMA) |
What are the real risks — and who should consider proceeding?
Miscarriage risk is the question families ask first, and the honest answer is that the risk is real but lower than popular perception often suggests.
ACOG Practice Bulletin No. 162 quantifies the procedure-related pregnancy loss rate — meaning risk above background — at approximately 1 in 769 for amniocentesis (roughly 0.13%) and 1 in 455 for CVS (roughly 0.22%) in experienced hands. These are not the same as overall miscarriage rates at these gestational ages; they represent the additional risk attributable to the procedure itself. Some large contemporary series at high-volume centers have reported amniocentesis loss rates approaching 0.1%, suggesting the published ACOG estimate may even be slightly conservative when a skilled operator is involved.
CVS carries a modestly higher procedural risk for two compounding reasons: it is performed earlier in pregnancy, when background spontaneous loss rates are naturally higher, and approximately 2% of CVS results show confined placental mosaicism (CPM) — a finding in which the chromosomal abnormality exists only in the placenta, not in the fetus. When CPM is found, a follow-up amniocentesis is typically recommended to clarify the fetal karyotype, which means some patients end up having both procedures.
As for who should consider proceeding: ACOG Practice Bulletin No. 162 broadened the traditional high-risk indications. Today, any pregnant individual who wants a definitive chromosomal result should be offered the option, regardless of age or whether they fall into a classically high-risk category. Specific indications include an abnormal NIPT or serum screening result, a fetal structural anomaly found on ultrasound, advanced maternal age (35 or older at delivery), a prior affected pregnancy, parental carriage of a chromosomal rearrangement, or a family history of a monogenic disorder such as cystic fibrosis, sickle cell disease, or Tay-Sachs disease.
The ACOG miscarriage figures — 1 in 769 for amnio, 1 in 455 for CVS — represent additional risk above background. In high-volume centers, amnio rates may be closer to 1 in 1,000. Ask your provider about their center's specific procedural outcomes before deciding.
How are the results analyzed — and what does chromosomal microarray actually detect?
Retrieving the sample is only the first step; the analytic method applied to it determines how much information you receive.
Chromosomal microarray analysis (CMA) is now the ACOG-recommended primary analytic method for both CVS and amniocentesis specimens, per Practice Bulletin No. 162. CMA scans the entire genome simultaneously for submicroscopic deletions and duplications — called copy-number variants (CNVs) — that a standard karyotype cannot detect. A conventional karyotype shows only large chromosomal changes visible under a light microscope; CMA resolves abnormalities at the kilobase scale, identifying conditions like DiGeorge syndrome (22q11.2 deletion) that a karyotype would miss entirely. CMA also does not require cells to be cultured, which shortens the wait compared to a traditional karyotype.
In practice, many centers offer a tiered approach: rapid FISH (fluorescence in situ hybridization) tests for the most common aneuploidies — trisomies 21, 18, and 13, and sex-chromosome abnormalities — and typically returns results in 24 to 48 hours. CMA results follow in 5 to 10 business days with the comprehensive picture. When a structural fetal anomaly is detected on ultrasound and CMA comes back normal, exome sequencing of the amniotic fluid or CVS sample is an emerging option that can identify single-gene variants; ACOG does not yet endorse routine exome or genome sequencing outside of a specialized clinical or research context.
Results are used to confirm or rule out the suspected chromosomal finding, to guide obstetric management (including delivery timing, delivery hospital level, and neonatal subspecialty planning), and to give families the definitive information they need for counseling and decision-making. A genetic counselor — who can be provided through many diagnostic laboratories at no additional cost — plays a critical role in explaining what a result does and does not mean for the pregnancy and for future family planning.
CVS or amniocentesis: how to frame the decision with your provider
The most important variable in the CVS-vs.-amniocentesis decision is often gestational age. If you are still in the first trimester and want answers as early as possible — perhaps because early results allow more time for counseling, specialist consultation, or decision-making — CVS is the only option. If you are past 14 weeks, amniocentesis becomes available and is the more common choice at most centers.
A few other considerations are worth raising with your provider:
- Operator experience matters substantially. Procedure-related risk is strongly correlated with the number of procedures a clinician has performed. Ask how many CVS or amniocentesis procedures your provider or center performs annually, and what their center-specific complication rates are.
- Placental location affects CVS route. A posterior or fundal placenta may make transcervical CVS difficult; the transabdominal route or amniocentesis may be the preferred path.
- The 2% CPM rate with CVS. If a CVS result comes back mosaic, a follow-up amniocentesis is often recommended. Factor this into your timeline if gestational age is a concern.
- Your reason for testing. If the indication is confirming an abnormal NIPT result for trisomy 21, either procedure will provide that answer. If the indication is a structural anomaly on ultrasound, your MFM specialist may have a preference based on what additional analytic detail is needed.
Finally, it is entirely reasonable to decline diagnostic testing, choose to proceed based on screening results alone, or take time to discuss with a genetic counselor before deciding. ACOG frames the offer of diagnostic testing as expanding options, not mandating a particular path. Both procedures have a strong safety record in experienced hands, and the information they provide — definitive, actionable, and not obtainable any other way — remains the reason they continue to be offered as standard care in 2026.
Frequently asked
What is the difference between amniocentesis and CVS?
Both procedures retrieve fetal genetic material for a definitive chromosomal diagnosis, but they differ in timing and what they sample. CVS (chorionic villus sampling) is performed between 10 and 13 weeks of gestation and retrieves a small sample of placental tissue (chorionic villi), which is genetically identical to the fetus. Amniocentesis is typically done from 15 to 20 weeks and withdraws a small volume of amniotic fluid containing shed fetal cells. CVS gives you first-trimester answers; amniocentesis is performed in the second trimester. ACOG describes both as the gold standard for diagnostic certainty when a screening result is abnormal or when families want the most definitive available information. This is general information — always discuss your individual situation and gestational age with your provider.
What is the miscarriage risk from amniocentesis vs. CVS?
Per ACOG Practice Bulletin No. 162, the procedure-related pregnancy loss rate in experienced hands is approximately 1 in 769 (about 0.13%) for amniocentesis and 1 in 455 (about 0.22%) for CVS. These figures represent additional risk above the background loss rate that exists in all pregnancies at these gestational ages. Some large contemporary series performed at high-volume centers have reported amniocentesis loss rates as low as 0.1%, suggesting the ACOG estimate may be conservative when a skilled operator is involved. CVS carries a slightly higher rate partly because it is performed earlier, when background miscarriage rates are also higher. Discuss the specific loss rates at your center with your provider before deciding.
Who is a candidate for diagnostic prenatal testing?
ACOG Practice Bulletin No. 162 broadened the traditional indications considerably: any pregnant individual who wants a definitive chromosomal result should be offered the option, not only those in historically high-risk categories. Specific clinical indications include an abnormal NIPT or serum screening result, a structural fetal anomaly detected on ultrasound, advanced maternal age (35 or older at delivery), a prior pregnancy affected by a chromosomal condition, parental carriage of a chromosomal rearrangement or a known pathogenic variant, or a family history of a monogenic disorder such as cystic fibrosis or sickle cell disease. Contemporary OB/GYN summarizes the full ACOG indication list. Ultimately, diagnostic testing is elective — the goal is informed decision-making, not mandated testing.
What does a chromosomal microarray find that a standard karyotype misses?
A chromosomal microarray analysis (CMA) scans the entire genome for submicroscopic deletions and duplications — copy-number variants (CNVs) — that a standard karyotype cannot resolve. A karyotype shows large structural changes visible under a microscope (such as an extra chromosome 21 in Down syndrome), but it cannot detect tiny deletions or duplications at the kilobase scale. CMA does not require cells to be cultured, which speeds turnaround. ACOG Practice Bulletin No. 162 now recommends CMA — rather than karyotype — as the primary analytic method when CVS or amniocentesis is performed. Microarray results typically return within 5 to 10 days; rapid FISH for the common aneuploidies can return in 24 to 48 hours when urgency requires it.
What is confined placental mosaicism and why does it matter for CVS?
Confined placental mosaicism (CPM) occurs when a chromosomally abnormal cell line is present in the placenta but not in the fetus itself. Because CVS retrieves placental tissue (chorionic villi) rather than fetal cells directly, CPM can produce an abnormal CVS result even though the fetus carries a normal karyotype. According to the Cleveland Clinic, approximately 2% of CVS procedures yield a mosaic result attributable to CPM. When this happens, a follow-up amniocentesis — which samples amniotic fluid containing cells shed directly by the fetus — is often recommended to clarify whether the abnormality is truly present in the fetus. This is one of the key reasons CVS may require a second procedure, whereas amniocentesis samples fetal cells from the outset.
How long does it take to get results from amniocentesis or CVS?
Turnaround depends on which analytic method is used. Rapid FISH (fluorescence in situ hybridization) tests for the most common aneuploidies — trisomies 21, 18, and 13, and sex-chromosome abnormalities — and typically returns results in 24 to 48 hours when an urgent answer is needed. Chromosomal microarray analysis (CMA), now the ACOG-recommended primary method, generally takes 5 to 10 business days. A full standard karyotype with culture takes up to two weeks. Many centers run rapid FISH and CMA in parallel so that the most common abnormalities are ruled in or out quickly while the comprehensive microarray result is pending. Your provider will walk you through what your specific lab offers and how results will be delivered, typically through a genetic counselor. See ACOG's amniocentesis FAQ for additional procedural detail.