Source: https://guthlabs.ai/docs/instruments/gestational-age-edd-from-ultrasound/implementation/

instrument implementation / implementation-contract

# Gestational Age / EDD from Ultrasound implementation

A source-bound implementation contract for Gestational Age / EDD from Ultrasound; deployment and executable qualification remain separate.

Status: **VERIFIED_CLEAN**Lifecycle: **implementation-contract**

## Binding contract

This record describes what an executable surface must bind and validate. It does not claim that a public calculator, package, or production route is deployed.

**Record owner**: Guth Instruments

**Lifecycle**: verified-clean

**Evidence ID**: t4-264

**Effect**: Computation or source lookup only

## Input and validation requirements

The specification exposes two failure hazards: invalid calendar dates can produce undefined results unless rejected, and interpreting GA_at_scan as weeks instead of the formula's indicated days changes the EDD from 2025-02-16 to 2024-10-19. The formula also requires a leap-year calendar branch at February 29.

## Required corrections

No binding erratum is attached to this unique-name record. Implementers must still preserve the declared units, domain, and failure behavior.

## Inspectable interface fixture

The sample below is evidence for interface and validation tests, not a production implementation.

````
{
  "inputs": {
    "scan_date": "2025-01-15",
    "ga_at_scan_days": 84,
    "today": "2025-02-12"
  },
  "expected_outputs": {
    "current_ga_days": 112,
    "current_ga": "16 weeks 0 days",
    "revised_edd": "2025-07-30"
  },
  "working": "days_elapsed = 2025-02-12 − 2025-01-15 = 28 days. current GA = 84 + 28 = 112 days = 16 weeks 0 days. Remaining gestation = 280 − 84 = 196 days. EDD = 2025-01-15 + 196 days = 2025-07-30."
}
````

## Other records

[01**Overview**](<https://guthlabs.ai/docs/instruments/gestational-age-edd-from-ultrasound/>)[02**Mechanism**](<https://guthlabs.ai/docs/instruments/gestational-age-edd-from-ultrasound/mechanism/>)[04**Qualification**](<https://guthlabs.ai/docs/instruments/gestational-age-edd-from-ultrasound/qualification/>)

## Sources and evidence

These pointers bind the page to evidence. They do not grant authority or collapse verification into deployment.

- `VERIFIED_LEDGER.json`

- `evidence-tranche4/tranche4-results.json`