Enter two values (EFW g, AC mm, or FL mm) to see the percent size difference between twins and a plain-language band.
What this tool calculates
- Percent difference (“discordance”) between twins using the standard formula: (larger − smaller) ÷ larger × 100.
- Works with estimated fetal weight (EFW, g) or single biometry measures such as abdominal circumference (AC, mm) or femur length (FL, mm) as entered.
How we present the result
- A single percentage to one decimal place.
- A plain-language band to help frame follow-up (e.g., “very small” under ~10%, “significant” from ~20%, “high” at ~25% or more). Exact thresholds vary by service and clinical context.
Scope and limits
- Discordance is a screening metric to flag twins for closer review (growth curves, Dopplers, fluid, chorionicity-specific considerations). It is not a diagnosis.
- Services commonly use EFW-based discordance for decisions; single-measure discordance (AC/FL) can guide interim judgement but should be interpreted with full ultrasound context.
- Local protocols differ (e.g., action thresholds around ≥20–25% discordance, and different surveillance schedules for monochorionic vs dichorionic twins).
Sources
- RCOG Green-top Guideline No. 51 — Management of Monochorionic Twin Pregnancy (includes definition and thresholds for EFW discordance). rcog.org.uk/…/management-of-monochorionic-twin-pregnancy-green-top-guideline-no-51
Last updated: 6 Oct 2025
How to use this calculator
- Select the measurement type: EFW (g), AC (mm), or FL (mm).
- Enter Twin A and Twin B using the same units and same scan.
- Tap Calculate to get the discordance % and an interpretation band.
Formula: (larger − smaller) ÷ larger × 100.