Source: https://guthlabs.ai/docs/instruments/ovulation-fertile-window-estimator/implementation/

instrument implementation / implementation-contract

# Ovulation & Fertile Window Estimator implementation

A source-bound implementation contract for Ovulation & Fertile Window Estimator; 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**: t4-253

**Effect**: Computation or source lookup only

## Input and validation requirements

The exact-zero offset case produces an estimated fertile window beginning before the recorded LMP, with no specified input validation or rejection rule. The specification also has no physiological formula branch and leaves date-counting and window-endpoint conventions ambiguous; the branch test therefore exposes calendar handling rather than a biological regime switch.

## 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": {
    "last_period_date": "2025-01-01",
    "average_cycle_length_days": 28
  },
  "expected_outputs": {
    "estimated_ovulation_date": "2025-01-15",
    "fertile_window_start": "2025-01-10",
    "fertile_window_end": "2025-01-20",
    "next_period_estimate": "2025-01-29"
  },
  "working": "Ovulation offset = 28 − 14 = 14 days. 2025-01-01 + 14 days = 2025-01-15. Fertile-window start = 2025-01-15 − 5 days = 2025-01-10. Fertile-window end = 2025-01-15 + 5 days = 2025-01-20. Next period = 2025-01-01 + 28 days = 2025-01-29."
}
````

## Other records

[01**Overview**](<https://guthlabs.ai/docs/instruments/ovulation-fertile-window-estimator/>)[02**Mechanism**](<https://guthlabs.ai/docs/instruments/ovulation-fertile-window-estimator/mechanism/>)[04**Qualification**](<https://guthlabs.ai/docs/instruments/ovulation-fertile-window-estimator/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`