Twin Growth Discordance Calculator (% Difference)

Enter two values (EFW g, AC mm, or FL mm) to see the percent size difference between twins and a plain-language band.

Measurement type
Discordance uses the same formula for all three: (larger − smaller) ÷ larger.
Twin A
Twin B
Discordance
Formula: (larger − smaller) ÷ larger × 100. Use the same units for both twins.

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

Last updated: 6 Oct 2025

How to use this calculator

  1. Select the measurement type: EFW (g), AC (mm), or FL (mm).
  2. Enter Twin A and Twin B using the same units and same scan.
  3. Tap Calculate to get the discordance % and an interpretation band.

Formula: (larger − smaller) ÷ larger × 100.