📐 Fetal Measurement Reference · z-score & Clinical Meaning (US/MRI)
Compares a single fetal measurement against a gestational-age reference, returning a z-score and percentile (for mean ± SD parameters) or a threshold band (for ventricle/cisterna magna/nuchal fold/NT), with clinical direction.
Fetal Measurement Reference · z-score & Clinical Meaning (US/MRI)
Gestational age (wk)
GA days (optional 0–6) (d)
Measurement
Measured value (mm)
When to use
Enter GA, select the parameter, and enter the measured value in mm. For biometric parameters the tool gives median/SD, z-score and percentile; for cutoff parameters it returns a normal/abnormal band with workup suggestions.
How it works
z = (value − GA-median)/SD; percentile = Φ(z). Cutoff parameters: lateral-ventricle atrium ≥10mm abnormal (10–12 mild/13–15 moderate/>15 severe); cisterna magna 2–10mm normal; nuchal fold ≥6mm (15–22 wk); NT ≥3.0mm (11–13⁺⁶ wk).
Key points
- The embedded reference table is an estimated magnitude, not a validated local chart — definitive growth assessment uses GA-specific curves (INTERGROWTH-21st/Hadlock) with serial measurements. (original synthesis · not guideline verbatim)
- Threshold parameters carry different downstream pathways: ventriculomegaly triggers MRI + karyotype/microarray + TORCH, while an effaced cisterna magna points toward Chiari II/open spina bifida.
- A single abnormal soft marker (e.g. NF, short FL) neither confirms nor excludes aneuploidy; integrate with the full anatomic survey and screening.
References
- Snijders RJ, Nicolaides KH. Fetal biometry at 14-40 weeks' gestation. Ultrasound Obstet Gynecol. 1994;4(1):34-48.
- ISUOG Practice Guidelines: ultrasound examination of the fetal central nervous system. Ultrasound Obstet Gynecol. 2020;56(3):476-484.
Decision support for licensed clinicians only; not a substitute for clinical judgement, diagnosis or local protocols.