# Prenatal Genetic Screening and Diagnostic Testing

## Overview

Prenatal genetic screening and diagnosis have evolved rapidly with the advent of cell-free DNA (cfDNA) technology. All pregnant patients should be offered screening and/or diagnostic testing regardless of age or risk factors. A critical distinction must be understood from the outset: screening tests provide risk estimates characterized by sensitivity and specificity, whereas diagnostic tests -- chorionic villus sampling (CVS) and amniocentesis -- provide definitive answers. Genetic counseling is essential for informed decision-making and proper interpretation of results.

## Screening Modalities

### First-Trimester Combined Screening (11-13+6 weeks)

First-trimester combined screening uses three components: nuchal translucency (NT) measurement, serum PAPP-A, and free beta-hCG. This combination achieves detection rates of approximately 82 to 87% for trisomy 21 at a 5% false-positive rate, about 90% for trisomy 18, and roughly 80% for trisomy 13. The NT measurement requires a credentialed sonographer and a quality assurance program. An increased NT of 3.5 mm or more warrants further workup even if the karyotype is normal, because it is associated with cardiac defects and various genetic syndromes.

### Quad Screen (15-22 weeks, optimal 16-18 weeks)

The quad screen measures four analytes: AFP, hCG, unconjugated estriol (uE3), and inhibin A. Its detection rate for trisomy 21 is approximately 81% at a 5% false-positive rate. The quad screen also provides information about open neural tube defects (elevated AFP) and trisomy 18 (characterized by low AFP, low uE3, and low hCG). If first-trimester screening has already been performed, the quad screen should not be done independently, as combining independent screens without intent leads to unacceptably high false-positive rates.

### Sequential and Integrated Screening

Integrated screening combines first-trimester markers (NT plus PAPP-A) with second-trimester quad markers, delivering a single result after the second-trimester draw. This approach achieves a detection rate of approximately 96% for trisomy 21 at a 5% false-positive rate. Sequential screening reports first-trimester results immediately; if negative, second-trimester screening is added to produce a combined risk, achieving a step-wise detection rate of about 95%. Contingent screening triages first-trimester results into high-risk (proceed to diagnostic testing), low-risk (no further testing), or intermediate-risk (add second-trimester markers) categories.

### Cell-Free DNA (cfDNA) / Non-Invasive Prenatal Testing (NIPT)

cfDNA screening analyzes placental (trophoblast)-derived cell-free DNA fragments circulating in maternal blood and can be performed from 10 weeks of gestation. Its detection rates are impressive: 99.5% sensitivity and 99.9% specificity for trisomy 21, 97 to 99% sensitivity for trisomy 18, 91 to 99% sensitivity for trisomy 13, and variable performance (85 to 99%) for sex chromosome aneuploidies. The false-positive rate for trisomy 21 is less than 0.1%.

Several factors can cause false results. Confined placental mosaicism is the most common cause of false positives. A vanishing twin, maternal copy number variants, or occult maternal malignancy can also produce discordant results. Low fetal fraction (below 4%) causes test failure rather than a result. Low fetal fraction is more likely with early gestational age, high maternal BMI, and certain aneuploidies (especially trisomy 13 and 18).

ACOG now supports offering cfDNA to all patients regardless of risk category. However, cfDNA is not diagnostic -- positive results must always be confirmed with CVS or amniocentesis before any irreversible decision is made.

| Screening Modality | Gestational Age | Trisomy 21 Detection Rate | False-Positive Rate | Key Components |
|---|---|---|---|---|
| First-trimester combined | 11-13+6 weeks | 82-87% | 5% | NT, PAPP-A, free beta-hCG |
| Quad screen | 15-22 weeks | 81% | 5% | AFP, hCG, uE3, inhibin A |
| Integrated screening | First + second trimester | 96% | 5% | NT, PAPP-A + quad markers |
| Sequential screening | First + second trimester | 95% | 5% | Stepwise reporting |
| cfDNA (NIPT) | ≥10 weeks | 99.5% | <0.1% | Cell-free placental DNA |

### Expanded cfDNA Panels

Some laboratories offer expanded panels that screen for microdeletions such as 22q11.2 (DiGeorge), 1p36, Angelman, Prader-Willi, and Cri-du-chat syndromes. These expanded panels carry higher false-positive rates and lower positive predictive values (PPV) than standard trisomy screening. ACOG and SMFM do not currently recommend routine microdeletion screening due to the low PPV in the general population. The PPV depends heavily on condition prevalence: for trisomy 21 in a 35-year-old, PPV may reach 80%, but for a rare microdeletion, it may be less than 5%.

## Diagnostic Testing

### Chorionic Villus Sampling (CVS)

CVS is performed between 10 and 13 weeks and 6 days, using either a transcervical or transabdominal approach under ultrasound guidance. The procedure samples chorionic villi -- the cytotrophoblast layer provides a rapid result while the mesenchymal core is used for culture. Results include karyotype (7 to 14 days for culture), FISH (24 to 48 hours), and chromosomal microarray. The procedure-related pregnancy loss risk is approximately 1 in 500, significantly lower than the historically quoted 1 in 100 figure from older studies.

CVS has certain limitations. Confined placental mosaicism, occurring in 1 to 2% of cases, may yield results discordant from the true fetal karyotype. CVS cannot detect neural tube defects because no AFP measurement is obtained. If performed before 10 weeks, CVS has been associated with limb reduction defects, which is why it is no longer performed at such early gestational ages.

### Amniocentesis

Amniocentesis is performed at 15 weeks or later (typically 15 to 20 weeks) using a transabdominal, ultrasound-guided needle inserted into the amniotic cavity to sample amniocytes. Results include karyotype (10 to 14 days), FISH (24 to 48 hours), chromosomal microarray, and AFP level. The procedure-related loss risk is approximately 1 in 500 to 1 in 900. Compared to CVS, amniocentesis offers a definitive fetal karyotype without the concern for confined placental mosaicism, the ability to measure AFP, and a later-gestation option. Potential complications include leakage of amniotic fluid, chorioamnionitis (rare), and needle injury (rare).

| Feature | CVS | Amniocentesis |
|---|---|---|
| Gestational age | 10-13+6 weeks | ≥15 weeks |
| Approach | Transcervical or transabdominal | Transabdominal |
| Sample | Chorionic villi | Amniocytes |
| Karyotype turnaround | 7-14 days | 10-14 days |
| FISH turnaround | 24-48 hours | 24-48 hours |
| Procedure-related loss | ~1 in 500 | ~1 in 500-900 |
| AFP measurement | No | Yes |
| Confined placental mosaicism risk | 1-2% | Not applicable |

### Chromosomal Microarray (CMA)

Chromosomal microarray detects submicroscopic deletions and duplications that are not visible on standard karyotype. It is recommended as the first-line cytogenetic test when structural anomalies are detected on ultrasound, as it identifies clinically significant findings in an additional 6 to 7% of cases with anomalies and normal karyotype. A limitation is the identification of variants of uncertain significance (VUS) in approximately 1.5 to 2% of cases, which can cause parental anxiety.

## Counseling Frameworks

### Pre-Test Counseling

Effective pre-test counseling explains the difference between screening and diagnostic testing, discusses detection rates, false-positive rates, and the concept of positive predictive value, and explores the patient's values regarding what they would do with the information. Counseling should be non-directive, supporting patient autonomy. Patients should understand that screening is optional and that declining screening is a valid choice.

### Post-Test Counseling for Abnormal Results

When a screening test is abnormal, the result should be discussed as a risk estimate rather than a diagnosis, and diagnostic confirmation should be offered. When a diagnostic test confirms an abnormality, clear information about the condition, prognosis, and options should be provided. Options after a confirmed diagnosis include continuing the pregnancy with preparation, adoption planning, or pregnancy termination. Referral to a genetic counselor and appropriate specialists (such as a pediatric cardiologist or pediatric surgeon) is warranted, and families should be connected with support organizations for the specific condition.

## Special Populations

In patients of advanced maternal age (35 or older), the higher pretest probability increases the PPV of screening. Historically, these patients were offered diagnostic testing directly, but current practice allows them to choose between screening and diagnostic testing. In multiple gestations, cfDNA performance is reduced in twins, combined screening can be used, and zygosity affects interpretation. In egg donor pregnancies, the donor's age should be used for risk calculations, not the recipient's age. In IVF pregnancies, PGT results should be discussed in context, keeping in mind that PGT-A is not equivalent to prenatal diagnosis. In patients with obesity, higher test failure rates with cfDNA occur due to lower fetal fraction.

<image>
A decision tree flowchart for prenatal genetic screening and diagnostic testing. The tree starts with "Patient counseling at first prenatal visit: screening vs. diagnostic testing." It branches into three main paths: (1) Screening with cfDNA showing pathways for positive, negative, and no-call results; (2) Traditional screening (first-trimester combined or sequential) with positive screen leading to diagnostic testing options; (3) Direct diagnostic testing (CVS at 10-13 weeks or amniocentesis at >= 15 weeks). Each endpoint shows next steps including confirmatory testing, ultrasound surveillance, or genetic counseling referral.
</image>

<image>
A comparative illustration showing CVS and amniocentesis procedures side by side. The left panel depicts transcervical CVS with a catheter passing through the cervix to sample chorionic villi, with labeled placenta, chorion, and ultrasound probe positioning. The right panel shows amniocentesis with a needle passing transabdominally into the amniotic cavity, with labeled amniotic fluid, fetus, and ultrasound guidance. Both panels include gestational age windows and key technical details annotated around the illustrations.
</image>

<image>
An infographic comparing the performance characteristics of different prenatal screening modalities. Four columns represent: First-trimester combined screening, Quad screen, Integrated/Sequential screening, and cfDNA. For each modality, bar charts show detection rates for Trisomy 21, Trisomy 18, and Trisomy 13, along with false-positive rates. Below each column, the optimal gestational age window, components measured, and turnaround time are listed. A callout box highlights the critical distinction between high detection rate and positive predictive value.
</image>

## Key Clinical Pearls

A positive cfDNA is not a diagnosis -- it must always be confirmed with CVS or amniocentesis before any irreversible decision. PPV depends on prevalence: cfDNA for trisomy 21 in a 25-year-old has a PPV of approximately 50%, but in a 40-year-old it may reach 90%. A "no-call" or failed cfDNA result due to low fetal fraction is not reassuring and is actually associated with an increased risk of aneuploidy -- diagnostic testing should be offered. An increased NT with normal karyotype warrants fetal echocardiography, since 40% of fetuses with an NT above 3.5 mm have cardiac anomalies. Confined placental mosaicism is the most common reason for discordance between CVS results and the true fetal karyotype. Microarray should be offered as the primary cytogenetic test when structural anomalies are seen on ultrasound. Performing both first-trimester screening and the quad screen independently should be avoided, as this leads to unacceptably high false-positive rates.

## References
- ACOG Practice Bulletin No. 226: Screening for Fetal Chromosomal Abnormalities (2020)
- ACOG Committee Opinion No. 693: Counseling About Genetic Testing and Communication of Genetic Test Results (2017)
- SMFM Statement: Maternal Serum Cell-Free DNA Screening (2022)
- Norton ME et al. Cell-free DNA Analysis for Noninvasive Examination of Trisomy (NEXT study). N Engl J Med. 2015;372:1589-1597
- Wapner RJ et al. Chromosomal Microarray vs. Karyotype for Prenatal Diagnosis. N Engl J Med. 2012;367:2175-2184