Gestational Age / EDD from Ultrasound mechanism

Model scope

The arithmetic formulas are standard conditional formulas for propagating an already-established ultrasound gestational age and calculating a 280-day EDD. The exact unresolved ambiguity is whether the interface supplies a current/today date and whether GA_at_scan is entered in days, weeks-and-days, or another unit. The existing estimate-only and not-medical-advice labeling is appropriate, but the interface should also state that ultrasound dating inputs must come from an appropriate clinical method and should define valid GA ranges and date handling.

• GA_today=GA_at_scan+days_elapsed is standard only when GA_at_scan and days_elapsed are both expressed in elapsed calendar days; the specification does not define the required today/current-date input.
• EDD=scan_date+(280−GA_at_scan_days) is the standard 280-day gestational-age calculation conditional on a valid ultrasound-based GA at scan, but the specification does not provide a formula for deriving GA from CRL or other biometry.
• The claimed CRL/biometry capability is underspecified: the listed inputs contain only scan date and GA at scan, not CRL, biometric measurements, measurement units, or a reference dating chart/model.

Units and conventions

• GA_at_scan must be an integer or precisely defined fractional number of gestational days; if entered as weeks and days, it must be converted first, for example 12 weeks 3 days = 87 days.
• days_elapsed must mean calendar days from the scan date to the requested current date, not elapsed 24-hour periods across time zones or daylight-saving transitions.
• EDD is a calendar date calculated using 280 total gestational days, equivalent to 40 weeks.
• Date arithmetic must use a defined calendar, including leap-year handling and a date-only timezone-independent representation.

Inspectable vector

One of 4 source-bound review vectors:

{
  "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."
}

Failure boundary

BREAK_CLAIMED · SELF_FLAGGED

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.

Other records

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
• BOUNDARY_HAZARD_CATALOG.json