First‑Trimester Combined Screening for Fetal Aneuploidy

1. Universal Offer and Core Components

• All pregnant individuals, irrespective of maternal age, should be offered a first‑trimester combined screen (nuchal translucency ultrasound + maternal serum PAPP‑A and free β‑hCG or intact hCG) between 11 weeks 0 days and 13 weeks 6 days of gestation. [1][2]3
• An increased nuchal translucency (≥ 3 mm or ≥ 99th percentile for crown‑rump length) is significantly associated with Down syndrome and other chromosomal aneuploidies. 1
• Pregnancy‑associated plasma protein‑A (PAPP‑A) concentrations are typically reduced in pregnancies affected by Down syndrome. 1
• Free β‑hCG concentrations are elevated in Down‑syndrome pregnancies; free β‑hCG is analytically superior, although intact hCG is more commonly used in the United States due to assay availability. 1

2. Performance Characteristics

• The combined first‑trimester screen detects approximately 85 %–92 % of Down‑syndrome cases with a false‑positive rate of 1 %–5 %, outperforming maternal‑age‑only screening (≈ 30 % detection) and NT‑only screening (≈ 70 % detection). 3
• The same screening algorithm also provides risk estimates for trisomy 18 and trisomy 13. 1

3. Quality‑Assurance Requirements

• Nuchal‑translucency measurement exhibits notable inter‑ and intra‑observer variability; therefore, rigorous operator training and certification are mandatory. [1][2]4
• Sonography programs that do not meet established training and ongoing quality‑assurance standards should not offer NT screening. [1][2]

4. Patient Counseling and Invasive Diagnostic Options

• All patients should be informed that invasive diagnostic testing (chorionic‑villus sampling or amniocentesis) is available regardless of maternal age to definitively diagnose major fetal aneuploidies. [1][2]4
• Patients who decline any information about fetal chromosomal status may opt out of testing after appropriate documentation. [1][2]4
• When screening yields an elevated risk, patients must receive their adjusted risk estimate for Down syndrome and be supported in making autonomous decisions. [2][5]4
• Because age is no longer used as a cutoff for offering invasive testing, predetermined age‑based thresholds are considered outdated. [2][5]4
• Invasive diagnostic testing (CVS or amniocentesis) should be offered to any patient with an elevated risk to obtain a definitive chromosomal diagnosis. [1][2]4

5. Action Thresholds for Nuchal Translucency

• An NT measurement ≥ 3.5 mm, even when the overall screen is low‑risk or a normal fetal karyotype has been identified, should trigger a detailed anatomic ultrasound, fetal echocardiogram, or both. [2][5]4
• Markedly increased NT is linked with congenital heart defects, diaphragmatic hernias, skeletal dysplasias, and a spectrum of genetic syndromes. [1][4]

6. Second‑Trimester Follow‑Up

• Patients who have undergone first‑trimester screening and/or CVS should be offered maternal‑serum alpha‑fetoprotein (MSAFP) screening and/or a comprehensive anatomic survey between 16 – 20 weeks to detect neural‑tube defects. [2][5]4
• First‑trimester combined screening does not evaluate for open neural‑tube defects; dedicated second‑trimester assessment is required. [2][5]4

7. Optional Additional Markers

• Nasal‑bone assessment may be added to the first‑trimester protocol, but only clinicians with specific training and continuous quality‑assurance participation should perform it. [1][2][6][4]
• Absence of the fetal nasal bone is associated with Down syndrome, yet its utility as a population‑wide screening marker remains controversial. [1][4]

8. Special Considerations for Multiple Gestations

• First‑trimester combined screening can be applied to multifetal pregnancies, but patients must be counseled about its reduced sensitivity and higher false‑positive rates compared with singleton pregnancies. [2][5]4
• NT measurement alone is useful for twin screening but yields a higher positive‑screening rate. 1
• Combined ultrasound‑and‑serum screening in multifetal gestations is less sensitive because serum marker concentrations represent an average of both fetuses. 1

9. Common Pitfalls to Avoid

• Performing NT measurements without certified training and ongoing quality‑assurance leads to unreliable results and should be avoided. [1][2]4
• Relying on NT measurement without accompanying biochemical markers reduces detection of Down syndrome from 85 %–92 % to roughly 70 %. [1][3]
• Failing to provide second‑trimester MSAFP screening or an anatomic survey after a first‑trimester screen leaves neural‑tube defects undetected. [2][5]4

First Trimester Screening for Chromosomal Abnormalities

Core Screening Components

• The American College of Radiology recommends nuchal translucency (NT) measurement by ultrasound combined with two maternal serum biochemical markers: pregnancy-associated plasma protein A (PAPP-A) and free beta-human chorionic gonadotropin (free β-hCG) or intact hCG, performed between 11-14 weeks of gestation, for first trimester screening for chromosomal abnormalities 7, 8
• Nuchal translucency (NT) measurement is a fluid-filled space behind the fetal neck measured via ultrasound between 11-14 weeks, with increased NT (≥3 mm or ≥99th percentile) significantly associated with Down syndrome and other aneuploidies 7, 9
• PAPP-A (pregnancy-associated plasma protein A) is typically reduced in Down syndrome pregnancies 7, 10
• Free β-hCG or intact hCG is elevated in Down syndrome, with free beta-hCG considered superior though intact hCG is more commonly used in the United States due to limited access 7, 10

Performance Characteristics

• The combined first trimester screening approach detects approximately 90% of Down syndrome cases with a 5% false-positive rate, which is substantially superior to maternal age alone (30% detection) or second-trimester serum screening (65% detection) 9
• The detection rate specifically for trisomy 21 using combined first trimester screening is approximately 70% when NT is used alone, but increases to 85-90% when combined with both biochemical markers 7

Timing and Technical Requirements

• The optimal gestational age window for NT measurement is 11 weeks 0 days to 13 weeks 6 days 7, 9
• Crown-rump length should be 45-84 mm for accurate NT measurement 9

Additional Markers (Optional)

• Nasal bone assessment can be incorporated into first trimester screening protocols but should be limited to clinicians with specific training and ongoing quality assurance participation 7
• Absence of the nasal bone is a powerful marker of aneuploidy and improves screening algorithms when added to the standard three markers 9

Quality Assurance Requirements

• NT measurement has considerable inter- and intra-observer variability, making ongoing quality assessment essential 9
• Operators must be appropriately trained and participate in external quality assurance programs 9

Screening Beyond Trisomy 21

• The combined first trimester screening also provides risk assessment for trisomy 18 and 13 7, 9
• Increased NT is associated with congenital heart defects, diaphragmatic hernias, skeletal dysplasias, and genetic syndromes 7

Multiple Gestations

• For twin pregnancies, NT measurement alone is useful for screening but is associated with a higher positive screening rate 7
• Combined ultrasound and serum screening for multifetal gestations remains less sensitive than in singleton pregnancies 7, 8

Critical Action Thresholds

• When NT measures ≥3 mm, genetic counseling and invasive diagnostic testing should be promptly offered 11
• Approximately one-third of fetuses with NT thickness above this threshold will have a chromosomal abnormality, with half being trisomy 21 11

Patient Counseling Requirements

• All women regardless of maternal age should be offered the option of invasive diagnostic testing (CVS or amniocentesis) to definitively identify all major fetal aneuploidies 7, 8
• Women who do not want any information regarding fetal chromosomal status should not be required to undergo testing after appropriate documentation 7