Enter Measurements

Range supported by the original calculator: 2–85 mm.

Leave blank to calculate gestational age (and expected NT) from CRL only.

Used to compute EDD from the CRL-based gestational age.

Results

EDD (based on CRL):
EDD = scan date + (280 days − CRL-based gestational age in days)

Measuring the CRL

CRL Measurement "Ultrasound measurement of the embryo or fetus in the first trimester (up to and including 13 6/7 weeks of gestation) is the most accurate method to establish or confirm gestational age" with accuracy of ±5–7 days. To measure the crown rump length (CRL) the image of the fetus or embryo  is obtained in a true midsagittal plane . The maximum length from cranium (top of the head) to caudal rump (bottom) is measured as a straight line. When possible , the mean of three discrete CRL measurements should be used for estimating the due date.
REFERENCE Methods for estimating the due date. Committee Opinion No. 700.American College of Obstetricians and Gynecologists. Obstet Gynecol 2017;129:e150–4.

Nuchal Translucency Clinical Interpretation

Clinical note: Nuchal translucency (NT) is a first-trimester ultrasound marker, not a diagnosis. Risk is dose-dependent and depends on the measured NT, crown rump length, whether the finding is isolated, genetic testing results, and follow-up anatomic and cardiac evaluation.

Risk stratification for increased NT

NT of 3.0–3.4 mm is often treated as a borderline increased range. NT of ≥3.5 mm is generally considered increased and historically approximates the 99th percentile in the 11–13+6 week window.

NT measurement Approximate chromosomal anomaly risk Pregnancy loss / fetal death Major fetal anomaly Approximate alive and well outcome
3.0–3.4 mm Increased above baseline; absolute risk varies by age, CRL, and screening results Usually lower than larger NT categories if isolated and follow-up is normal Consider targeted follow-up; risk increases if additional findings are present Most are normal, especially if genetic testing and anatomy/echo are normal
3.5–4.4 mm ~21% ~2.7% ~10% ~70%
4.5–5.4 mm ~33% ~3.4% ~18.5% ~50%
5.5–6.4 mm ~50% ~10% ~24% ~30%
≥6.5 mm ~64–65% ~19% ~46% ~15%

Table source: The approximate chromosomal anomaly, fetal loss, major anomaly, and alive/well outcome categories for NT 3.5–4.4 mm, 4.5–5.4 mm, 5.5–6.4 mm, and ≥6.5 mm are adapted from Souka AP, von Kaisenberg CS, Hyett JA, Sonek JD, Nicolaides KH. Increased nuchal translucency with normal karyotype. Am J Obstet Gynecol. 2005;192:1005–1021. PMID: 15846173, which summarizes outcome data from large first-trimester NT cohorts. The 3.0–3.4 mm row is included as a modern borderline-risk counseling category; exact risks vary by maternal age, CRL, screening results, and local testing strategy.

These values are approximate counseling ranges and should not be used as individualized risk prediction. Absolute risk varies with maternal age, CRL, referral population, whether cystic hygroma/hydrops is present, and results of diagnostic testing.

Conditions associated with increased NT

Trisomy 21Trisomy 18Trisomy 13Monosomy XTriploidyPathogenic CNVsRASopathies / Noonan spectrumCongenital heart diseaseStructural anomaliesPregnancy loss

  • Aneuploidy: Increased NT is strongly associated with common autosomal trisomies, monosomy X, and triploidy.
  • Copy-number variants: After normal karyotype, chromosomal microarray provides additional diagnostic yield, especially when NT is ≥3.5 mm or other anomalies are present.
  • Single-gene disorders: Consider RASopathy testing or exome sequencing when NT is large, cystic hygroma/hydrops persists, or structural anomalies are identified.
  • Congenital heart disease: Increased NT remains a recognized CHD risk marker even when chromosomes are normal; fetal echocardiography is generally recommended for NT ≥3.5 mm.
  • Neurodevelopment: Risk is mainly driven by an underlying genetic, structural, or cardiac diagnosis. When diagnostic testing, detailed anatomy, and fetal echocardiography are normal, prognosis is substantially improved.
Suggested evaluation pathway
  1. Confirm measurement quality: CRL 45–84 mm, true midsagittal profile, neutral fetal position, calipers on inner borders, and image magnification appropriate for NT assessment.
  2. Genetic counseling: Discuss that cfDNA is a screening test and does not evaluate all genetic causes of increased NT.
  3. Offer diagnostic testing: CVS or amniocentesis with karyotype and/or chromosomal microarray. Microarray is preferred when a structural anomaly is present and is often favored for markedly increased NT.
  4. Early anatomy follow-up: Consider early targeted anatomy evaluation around 14–16 weeks when feasible.
  5. Detailed anatomy ultrasound: Perform comprehensive anatomy ultrasound at 18–22 weeks.
  6. Fetal echocardiography: Recommend fetal echocardiography, particularly for NT ≥3.5 mm, persistent increased NT, cystic hygroma/hydrops, abnormal ductus venosus, tricuspid regurgitation, or any structural concern.
  7. Additional genetic testing: Consider RASopathy panel and/or prenatal exome sequencing when karyotype/microarray are normal but NT is large, persistent, or accompanied by anomalies.
EMR-ready counseling statement
Nuchal translucency interpretation is not available until CRL and measured NT are entered and calculated. If measured NT is not entered, document CRL-based gestational age only; no NT percentile or increased-NT counseling statement is generated.
References
  1. Souka AP, von Kaisenberg CS, Hyett JA, Sonek JD, Nicolaides KH. Increased nuchal translucency with normal karyotype. Am J Obstet Gynecol. 2005;192:1005–1021. PMID: 15846173.
  2. Grande M, Jansen FAR, Blumenfeld YJ, Fisher A, Odibo AO, Haak MC, Borrell A. Genomic microarray in fetuses with increased nuchal translucency and normal karyotype: a systematic review and meta-analysis. Ultrasound Obstet Gynecol. 2015;46:650–658. PMID: 25900824.
  3. ACOG Practice Bulletin No. 226. Screening for Fetal Chromosomal Abnormalities. Obstet Gynecol. 2020;136:e48–e69. PMID: 32804883.
  4. ISUOG Practice Guidelines: updated fetal cardiac screening. Ultrasound Obstet Gynecol. 2023;61:788–803.
  5. Di Girolamo R, et al. Whole exome sequencing in fetuses with isolated increased nuchal translucency: systematic review and meta-analysis. 2023. PMID: 37019452.