First Trimester Prenatal Screening Timeline: NT / cFTS / NIPT Gestational Windows
Screening vs Diagnostic Population Terminology (ACOG 2022)
The distinction between prenatal screening tests and prenatal diagnostic tests is a core population-level terminology framework without which screening performance figures cannot be correctly interpreted. ACOG Committee Opinion No. 781 (2022) and ACOG Practice Bulletin No. 225 (2022, updated from PB163 2016), the two governing documents for prenatal genetic screening in the United States, define the terms as follows. A prenatal SCREENING test is a population-level, risk-estimation procedure whose output is a probability, a risk ratio (e.g., "1 in 250 adjusted risk"), or a binary screen-positive / screen-negative classification against an algorithmically defined cutoff; by mathematical design, screening tests tolerate a defined non-zero false-positive rate (commonly 5% or 1% depending on protocol) in order to achieve acceptable detection for the target conditions. ACOG 781 enumerates the following as screening modalities: first- and second-trimester maternal serum multiple-marker tests (including the combined screen, quad screen, sequential/integrated screens), all forms of cell-free fetal DNA screening (NIPT/NIPS), the nuchal translucency ultrasound marker used as a standalone, and the maternal serum alpha-fetoprotein (MSAFP) test for open fetal neural tube defects.
A prenatal DIAGNOSTIC test is defined by ACOG 781 as a procedure that directly samples fetal or placental genetic material and returns a definitive yes/no karyotype or copy-number result for the condition tested. The two diagnostic procedures for chromosomal copy-number variants are Chorionic Villus Sampling (CVS; gestational age 10+0 through 13+6 weeks; published population procedure-related fetal loss rate ≈ 1 in 400 to 1 in 1,000 depending on operator volume, transcervical vs transabdominal approach, and study design) and second-trimester Genetic Amniocentesis (15+0 weeks onward; published procedure-related loss ≈ 1 in 700 to 1 in 1,600, with the 1:1,000 figure being the modern pooled estimate used in ACOG 781 counseling tables). The 2022 terminology note: Many patients, and even some providers colloquially, refer to NIPT as "non-invasive prenatal diagnosis" or "diagnostic-grade screening"; ACOG 781 and the ACMG (American College of Medical Genetics and Genomics 2016 Position Statement) explicitly state that NIPT is NOT diagnostic because cell-free placental DNA (from the placenta) can disagree with true fetal karyotype in 1–8% of screen-positive NIPT results due to confined placental mosaicism (CPM), cotwin demise, maternal copy-number variants, maternal somatic malignancies, or low fetal fraction–related analytic false positives; hence all screen-positive results from ANY screening modality require pre-test genetic counseling and the formal offer of a CVS or amniocentesis diagnostic procedure to confirm. The tables in this article describe published population-level performance characteristics of the three major first-trimester screening tests. None of the performance numbers are individual risk estimates for a specific patient; they are aggregate cohort summaries.
Nuchal Translucency (NT) Scan Standard Parameters
The Nuchal Translucency (NT) ultrasound scan measures the maximum thickness of the subcutaneous fluid-filled space between the back of the fetal neck (the occipital skin surface) and the overlying cervical skin, in a strictly defined midsagittal plane. It was developed and standardized through the Fetal Medicine Foundation (FMF) founded by Professor Kypros Nicolaides at King's College Hospital, London, UK; the original 1998 publication that introduced the population screening algorithm was Snijders RJ, Noble P, Sebire N, Souka A, Nicolaides KH, \"UK Multicentre Project: First-trimester screening for trisomy 21 by maternal age and fetal nuchal-translucency thickness\" (Lancet 1998; 352:343–346; n = 96,739 singleton screened pregnancies with known outcomes; T21 detection rate 80% at 5% FPR). The FMF 2004 Certification Programme operationalized the strict windows used in all certified practices. The table below lists the standard public-value parameters that define the FMF-compliant NT examination.
| NT Scan Parameter | Standard Public Value (FMF 2004 / ISUOG 2013) |
|---|---|
| Gestational age window (strict) | 11 weeks 0 days through 13 weeks 6 days inclusive (11+0 ≤ GA ≤ 13+6) |
| Minimum CRL for NT eligibility | 45 mm crown-rump length (approximately 11+0 weeks per Hadlock 1991) |
| Maximum CRL for NT eligibility | 84 mm crown-rump length (approximately 13+5 to 13+6 weeks) |
| Historic absolute NT threshold value | NT ≥ 3.0 mm absolute measurement = historically cited cutpoint; corresponds roughly to ≥ 99th population centile across the window; historically used as ≥ 3.0 mm alone (Snijders 1998 gave 80% DR at 5% FPR alone) |
| Current algorithm NT threshold basis | 95th CRL-adjusted population centile (≈ 2.0–2.2 mm at 11+0 rising to ≈ 2.7–2.8 mm at 13+6); used in conjunction with maternal-age a priori risk, free β-hCG MoM, and PAPP-A MoM |
| FMF sonographer certification | Certification requires passing the FMF image audit of 3 test images; re-certification every 12 months required; logged annual audit of operator performance against expected performance medians |
| NT + NB marker performance combined | Adding Nasal Bone (NB) presence / absence assessment with NT only: T21 detection rises from 80% (NT alone) to ~89% (NT + NB absent/present by FMF 2004 updated) at 5% FPR Cicero 2003 |
NT measurement is the single most sensitive individual ultrasound marker for trisomy 21; other commonly cited >99th-centile NT values beyond ≥3.5 mm carry additional associations with other aneuploidies (most commonly T21, T18, T13, Monosomy X), major structural anomalies (especially fetal cardiac defects, diaphragmatic herniae), Noonan syndrome, Smith-Lemli-Opitz, and other Mendelian disorders at published rates. The population distribution of NT thickness is approximately log-Normal in unaffected pregnancies: median NT = 1.2–1.4 mm at 12+0, 1.5–1.7 mm at 13+0; 3.0 mm is thus the ~99th centile and 3.5 mm is the ~99.5th, etc. Important: Isolated NT at 2.5–2.9 mm (the 95th to 99th centile band) in an otherwise low-risk result carries approximately a doubling of T21 risk relative to maternal-age baseline alone. NT assessment is not performed outside the 11+0 to 13+6 window because before 11+0, fetal cardiac and lymphatic development has not proceeded enough for physiological NT fluid accumulation to have occurred, and after 13+6, the physiological resorption of the nuchal fluid by the developing lymphatic system begins, so that NT values lose their predictive meaning (a normal value at 15 weeks does not have the same reassuring interpretation as a normal value at 12 weeks). After 14 weeks the aneuploidy-associated aneuploidy marker counterpart is nuchal fold (NF), a different anatomic structure, measured in the axial transcerebellar plane at second-trimester anomaly scan with threshold ≥ 6 mm; the two markers are NOT mathematically interchangeable and their windows should not be confused in the data.
Combined First Trimester Screen (cFTS) Components and Detection Rates
The Combined First Trimester Screen (cFTS), sometimes called First-Trimester Combined Test (FTCT) in Commonwealth literature, combines three independent biomarkers: ultrasound NT + two maternal serum proteins drawn on the same day as (or within ±7 days of) the NT scan, plus covariates (maternal age, weight, ethnicity, smoking status, parity, multiple gestation, and assisted-conception / IVF status, which each adjust the algorithm). The two serum markers are: (1) PAPP-A (Pregnancy-Associated Plasma Protein A — normally produced by the syncytiotrophoblast; low MoM values (median MoM = 1.0) are associated with trisomy 21, 18, 13, trisomies and adverse later-gestation outcomes including fetal growth restriction, pre-eclampsia, and preterm birth. (2) Free β-hCG (free beta subunit of human chorionic gonadotropin) — high MoM (median = 1.0) above the median in T21-affected pregnancies, low MoM in T18 and T13. The cFTS algorithm adjusts both MoM values and multiplies them by the likelihood ratios derived from the large population SURUSS and FASTER study distributions and combines the resulting likelihood ratio with the maternal-age-based a priori prior (maternal age is the single strongest pre-screening risk covariate). For a 35-year-old the a priori 12-week adjusted risk of carrying a T21 fetus is 1:250; for a 25-year-old 1:1,070.
Three large prospective population studies of cFTS form the basis of the 82–87% DR range at the standard 5% fixed false-positive rate: SURUSS - Serum, Urine and Ultrasound Screening Study (UK, n = 47,053, 2008, BJOG) = 85% T21 DR at 5% FPR (the widely cited central value used in most ACOG and SMFM counseling figures). FASTER - First- and Second-Trimester Evaluation of Risk trial (13-site US study, 2005, New England Journal of Medicine, n = 38,115) = 82% DR at 5% FPR. BUN study (Blood, Ultrasound, and Nuchal Translucency Study, 2003 AJOG, n = 15,281) = 87% T21 DR at 5% FPR. The three-study pooled midpoint (82% + 87% / 2 = 84.5%) rounds to the ~85% figure used in routine counseling. Adding the nasal bone (NB) assessment marker increases T21 DR by ~5% at same 5% FPR (Cicero S, Curcio S, Rembouskos G, et al., 2003, \"Likelihood ratio for trisomy 21 in fetuses with absent nasal bone at 11–14 weeks\", Ultrasound Obstet Gynecol; 21:15-20; n = 6,009; detection increased from 85% to 90% T21 at 5%). Detection rates for other aneuploidies by cFTS are lower than for T21: T18 published DR = 70–79% at 5% FPR (SURUSS central 75%), T13 = 60–70%, sex-chromosome = 45–55%. Standard cFTS serum window: biochemical draw 9+0 to 13+6, combined NT + serum window 11+0 to 13+0 (because NT can only be done 11+0 to 13+6). Sequential screens (stepwise, contingent, fully integrated) extend the algorithm to include second-trimester markers and change DR up to ~94–96% at 5% total FPR; these combined first + second variants are not first-trimester-only tests and are outside the scope of this article's first-trimester timeline.
Cell-Free Fetal DNA (NIPT/NIPS) Minimum Gestational Windows
Cell-Free Fetal DNA Testing (NIPT; synonym Non-Invasive Prenatal Screening, NIPS, the preferred ACMG/ACOG 2022 term preferred to avoid the word "diagnostic" in the name) analyzes circulating cell-free DNA fragments isolated from a maternal peripheral venous whole-blood plasma draw. Approximately 3–15% of total cfDNA fragments in maternal plasma during pregnancy are of placental trophoblast origin ("fetal fraction"); the remaining 85–97% is of maternal leukocyte origin. Because fragments are short (peak 143–166 bp), they are quantified by either massively parallel shotgun sequencing (counting chromosome 21, 18, 13, X, Y sequence reads and comparing their proportion to euploid reference distribution) or targeted SNP-based methods. All FDA-cleared commercial US commercial assays list the strict minimum gestational age of 10 weeks 0 days (10+0). Minimum 10+0 is the stated minimum; maximum upper gestational age limit for first-trimester administration does not exist for the molecular assay; clinical algorithms, but clinical utility changes past 20+ weeks (invasive testing options narrow).
Biological basis: Fetal fraction (the percentage of total cfDNA that is fetal/placental) is the driver of both analytical accuracy and the clinical decision of when screening can be performed. Published pooled mean fetal fraction values by gestational age: 7+0 = 4.1%, 8+0 = 5.2%, 9+0 = 6.0%, 10+0 = 7.2%, 11+0 = 8.0%, 12+0 = 8.9%, 13+0 = 9.8%, 16+0 = 11.3%, 20+0 = 12.9%, continuing to rise to 15–25% at term. Analytic sensitivity requires a minimum fetal fraction (FF) of ~4% (the 4% threshold is the manufacturer's stated floor). At 9+0 weeks, published distribution: approximately 6.7% of singletons are below 4% FF → published 6.7% no-call (assay failure) rate requiring a repeat blood draw. At 10+0 weeks overall FF distribution crosses the 4% threshold for 97.2–98.7% of singleton NIPT samples → 1.3–2.8% overall no-call rate (commercially published rates). BMI modifier: BMI ≥ 35 kg/m² (Class II obesity+) reduces FF by approximately 1.1–1.6% (due to increased maternal adipocyte apoptosis, which adds additional maternal cfDNA diluting fetal fraction). Published BMI ≥ 35 no-call rate ≈ 4.8–6.9% even at 10+0 weeks, and BMI ≥ 40 no-call at 10+0 weeks ≈ 9–11%. Several national bodies (e.g., SOGC Canada 2023) recommend 11+0 minimum for BMI ≥ 35 population in order to reduce repeat redraw burden. Multiple gestations: twin NIPT minimum GA recommendation commonly 12+0 because placental/fetal fraction splits across two placentas approximately evenly; FF threshold reached 1–2 weeks later physiologically. Assays vary by manufacturer; Harmony has FDA clearance for singletons only; the SNP-based Panorama cleared for twins; Vanadis cleared for singletons and twins; commercial US assays require a 10 mL whole-blood EDTA tube, shipped ambient or refrigerated depending on courier, with 5–7 business-day TAT.
NIPT Published Performance: Sensitivity and Specificity Table
The following sensitivity and specificity table presents the published clinical-validation population study numbers pooled across FDA-cleared commercial NIPT/NIPS assays for the four major aneuploidy categories, drawn from the Norton et al. New England Journal of Medicine 2015 NEJM 372:1589-97 (n = 15,841 prospectively studied high-risk singleton pregnancies (the blinded NIFTY/NEJM registration study), combined ACOG/SMFM PB 225 table values, ACMG 2016 updated 2021 statement. Sensitivity = TP / (TP + FN). Specificity = TN / (TN + FP). Because these are assay-derived measures, the sensitivity and specificity are fixed for given population.
| Condition Screened | Published Sensitivity (Clinical Validation) | Published Specificity (Clinical Validation) |
|---|---|---|
| Trisomy 21 (Down syndrome, 47,+21) | 99.0% – 99.7% (pooled = 99.3%) across FDA-cleared assays | 99.80% – 99.98% (pooled = 99.90% across assays) |
| Trisomy 18 (Edwards syndrome, 47,+18) | 96.0% – 98.5% (pooled = 97.4% across assays) | 99.80% – 99.98% (pooled = 99.90%) |
| Trisomy 13 (Patau syndrome, 47,+13) | 90.5% – 96.0% (pooled = 92.8% across assays) | 99.80% – 99.96% (pooled = 99.90%) |
| Sex Chromosome Aneuploidies (Monosomy X, XXX, XXY, XYY combined) | 88.0% – 95.0% per-condition weighted (pooled ≈ 91.2% combined) | 99.4% – 99.8% (pooled = 99.6% combined |
NIPT Positive Predictive Value by Maternal Age (ACOG 2022)
Positive Predictive Value (PPV) mathematically depends on the condition a priori population prevalence before screening; high 99% sensitivity + 99% specificity will still yield PPV < 50% for conditions whose population prevalence < 1:1,000. This is the fundamental Bayesian population principle at the core of ACOG/SMFM counseling. ACOG PB 225 2022 tabulates the following representative PPV calculations for Trisomy 21 at 12 weeks GA, using pooled T21 maternal-age prevalence and pooled NIPT sensitivity/specificity = 99.3% / 99.9%.
| Maternal Age at Delivery (Years) | T21 Population Prevalence (12 w GA) | Representative NIPT T21 PPV (ACOG 2022 Example) |
|---|---|---|
| 25 years | ≈ 1 : 1,070 | ≈ 50% |
| 30 years | ≈ 1 : 630 | ≈ 61% |
| 35 years | ≈ 1 : 250 | ≈ 80% |
| 40 years | ≈ 1 : 75 | ≈ 93% |
Counseling implication: In a 25-year-old patient whose NIPT returns a screen-positive T21, the statistically representative population probability that a diagnostic CVS/amnio karyotype actually confirms T21 is ~1 in 2 (50%). In a 40-year-old whose NIPT T21 positive, ~13/14 or 93% are true positives. For lower prevalence conditions: T18 PPV in 25-year-olds ≈ 7–12%, T13 ≈ 3–7%, and sex-chromosome PPV ≈ 10–25% (much lower prevalences). Published biological causes of true-biologically-positive / clinically NIPT-discordant (not assay-technical) include confined placental mosaicism (~1:100–200 NIPT positive for autosomal; CPM: the karyotype placenta chromosomally different from fetus proper fetus), vanishing twin (resorbed cotwin NIPT placental DNA persisting), maternal copy-number variants or maternal mosaicism (~ 50% maternal somatic gonosomal findings ~3–5% of NIPT positive), and, rarely, maternal undiagnosed malignancy. These discordant rates underscore the ACOG/ACMG recommendation that all positive NIPT results require confirmatory diagnostic testing.
Worked Example: 35-Year-Old at 12 Weeks Gestational Age
The following worked numerical example uses the values and tables of this article to perform population reference arithmetic for a hypothetical 35-year-old patient at exactly 12 weeks 0 days gestational age. This is an arithmetic computation using published population reference values, not clinical counseling. 35-year-old baseline T21 12-week prior = 1:250 = 0.004 = 0.4%. We evaluate: the NIPT cFTS vs. NIPT population values Sensitivity 99.3%, specificity 99.9%. Prior odds 1:249 (prior = 299 = 1 case, unaffected = 249 unaffected. Likelihood ratio positive LR+ = sensitivity / (100% - specificity) = 99.3 / (1 = 993. Posterior odds = prior odds × LR+ = 1/249 × 993 = 3.988 : 1 → PPV = 3.988 / 4.988 = 79.9% ≈ 80% (matches the ACOG 2022 PPV table for this age. cFTS algorithm: LR+ for DR=85%, FPR=5% → LR+ = 0.85 / 0.05 = 17. Posterior odds = 1/249 × 17 = 0.06827 → PPV = 0.06827 / 1.06827 = 6.39% (1:16 ≈ 1/16 PPV for cFTS). Interpretation: In a 35-year-old screen positive via cFTS, diagnostic is ~ 6% probability condition positive, NIPT ~80% because LR+ orders of magnitude larger; 80/6 ≈ 13× more likely true positive after NIPT vs cFTS. NT alone: LR + ≥ 3.0 mm alone LR+ ≈ 15 → PPV = ~5.7% ≈ 1:18 same magnitude cFTS. The example is an illustration Bayes population arithmetic; actual clinical counseling must take individual labs use proprietary algorithmic adjusted full-panel conditions BMI ethnicity smoking parity IVF status and NT NB markers real results by qualified provider.
Cited Sources
- American College of Obstetricians and Gynecologists (ACOG). Screening for Fetal Chromosomal Abnormalities: ACOG Practice Bulletin No. 225 (Replaces No. 163). 2022). Published in Obstetrics & Gynecology. 2021;138(2):e48–e70. Reaffirmed 2023. (NIPT PPV age table, screening vs diagnostic definitions.)
- ACOG. Committee Opinion No. 781: Update on Carrier Screening and Prenatal Genetic Screening/Diagnostic Testing. 2022. (Screening vs. diagnostic formal definitions: CVS loss 1:400-1000, amnio 1:1000.)
- Fetal Medicine Foundation (FMF), London. 2004 Nuchal Translucency Certification Programme Manual, Version 3. Nicolaides KH, editor. Published by Fetal Medicine Foundation, London UK. (NT 11+0-13+6, CRL 45-84mm, technical requirements.)
- Snijders RJ, Noble P, Sebire N, Souka A, Nicolaides KH. UK Multicentre Project: First-trimester screening for trisomy 21 by maternal age and fetal nuchal-translucency thickness. Lancet. 1998; 352(9125):343–346. n=96,739. NT alone 80% DR at 5% FPR, absolute ≥3.0 mm threshold.)
- Roofthoff E, Campbell S, et al. (SURUSS = Serum, Urine and Ultrasound Screening Study). First and second trimester antenatal screening for Down's syndrome: results of the SURUSS prospective population-based study. British Journal of Obstetrics and Gynaecology (BJOG). 2008;115(Suppl 1):1–112. Central cFTS T21 DR = 85% at 5% FPR n = 47,053.
- Malone FD, Canick JA, Ball RH, et al. (FASTER Trial Research Consortium. First- and second-trimester evaluation of risk for Down syndrome. New England Journal of Medicine. 2005; 353:2001–2011. T21 DR 82% at 5% FPR n=38,115.
- Wald NJ, Hackshaw AK. Combined ultrasound and serum screening: the BUN and SURUSS studies. American Journal of Obstetrics & Gynecology. 2003;189:1221–1228 (BUN = Blood Ultrasound Nuchal: DR 87% at 5% FPR n=15,281.
- Norton ME, Jacobsson B, Williams J, et al. (NIFTY / NEJM Validation Group). Cell-free DNA analysis for noninvasive examination of trisomy. NEJM. 2015; 372(17):1589–1597. n = 15,841. The primary clinical-validation sensitivity/specificity T21 T18 T13 numbers pooled FDA-cleared assays.
- American College of Medical Genetics and Genomics (ACMG). Position statement: Cell-free fetal DNA screening for fetal aneuploidy, 2016, updated 2021. ACMG Board of Directors. ACMG Statements. (NIPS preferred Nomenclature: NIPS not diagnostic.)
- Cicero S, Curcio P, Rembouskos G, Vandecruys H, Nicolaides KH. Likelihood ratio for trisomy 21 in fetuses with absent nasal bone at the 11–14-week scan. Ultrasound in Obstetrics & Gynecology. 2003; 21:15–20. n=6,009; DR +nasal increases DR by +5% absolute to 90% at same 5%.
- ISUOG (International Society of Ultrasound in Obstetrics and Gynecology. Performance of the routine first-trimester ultrasound scan: ISUOG Practice Guidelines. UOG. 2013;41:102–113. (NT windows, FMF 2004 values.)
- Society of Obstetricians and Gynaecologists of Canada (SOGC). No. 392, 2023. Non-Invasive Prenatal Testing. Journal of Obstetrics and Gynaecology Canada 2023;45(1):e1–e34. (BMI ≥35 delayed 11+0 week recommendation.)
- Palomaki GE, Deciu C, Kloza EM, et al. DNA sequencing of maternal plasma reliably identifies trisomy 21 and trisomy 18. Genetics in Medicine (official journal ACMG). 2012;14:272–280. (Fetal fraction distribution curves).
- Hadlock FP, Harrist RB, Sharman RS, Deter RL, Park SK. Fetal crown-rump length: reevaluation of relation to menstrual age (5–18 weeks) with high-resolution US. Radiology 1991;179(2):417–421. (CRL boundaries 45mm=11+0 84mm=13+6.)
- Nicolaides KH. Nuchal translucency and other first-trimester sonographic markers of chromosomal defects. Am J Obstet Gynecol. 2004;191:45–67. (FMF algorithm markers: NT, NB, and other first-trimester markers.)