Ovulation Calculator API

About the Ovulation Calculator API

Ovulation Calculator estimates when ovulation is likely in the current cycle from two inputs. Works as an ovulation date and cycle calculator: send the first day of the last period and the average cycle length. It uses the common calendar method, counting back from the expected start of the next period, and returns dates in YYYY-MM-DD format that drop straight into a fertility calendar.

Fertility tracking and cycle tracking apps, women's health content sites and family planning tools use it to show an estimated ovulation date, the fertile window and the days the calculator marks as most fertile, without maintaining the date arithmetic themselves.

The Free plan returns the estimated ovulation date with the inputs echoed back and a disclaimer. Paid plans add the fertile window and most fertile days, a day-by-day fertile days list with levels, the expected next period, the length of each cycle phase, and a current status showing where today falls in the cycle and how many days remain until ovulation and the next period. All results are estimates, not medical advice.

How it works

The calculation is pure date arithmetic with no medical model behind it. It assumes a fixed 14-day luteal phase, so ovulation is placed cycle_length minus 14 days after last_period: day 14 for the default 28-day cycle, day 7 for a 21-day cycle and day 21 for a 35-day cycle, the accepted range. Irregular cycles are not modelled, and the response includes a disclaimer field saying the figures are estimates.

last_period is a date in YYYY-MM-DD format, read as a calendar date with no time-zone shift, so the dates returned are the same calendar days you would count by hand. An unparseable date returns a 400.

fertile_window runs from five days before the estimated ovulation date through that date, six days in all. most_fertile_days is the last three of those days. fertile_days lists the six days individually, each with day_relative_to_ovulation from -5 to 0 and a fertility_level of medium for the first three and high for the last three. next_period.date is last_period plus cycle_length. cycle_phases reports the follicular phase as cycle_length minus 14 days, ovulation as one day and the luteal phase as 14.

current_status compares those dates with the server's current date: the phase is Follicular Phase before the window, Fertile Window inside it, Luteal Phase until the next period, and Menstruation once that date has passed, however long ago. The day counts go negative when a date is in the past. On the Free plan only ovulation.date, the echoed inputs and disclaimer are returned; all other fields are null.

What people use it for

Fertility Tracking

Build fertility tracking applications with accurate ovulation predictions based on cycle length and last period date

Conception Planning

Calculate conception probability windows and most fertile days to help users plan or prevent pregnancy

Menstrual Health

Create menstrual health apps with ovulation tracking, fertile window identification, and cycle insights

Family Planning

Provide family planning tools with science-based ovulation calculations and fertility awareness features

Ways to call it

One endpoint, many ways in — REST with JSON, XML, YAML and CSV, plus GraphQL and an MCP interface for AI agents.

JSON

Default REST response

XML

Markup format

YAML

Human-readable

CSV

Tabular export

Beta

GraphQL

Query language

New

MCP

For AI agents

Request parameters — GET /v1/ovulationcalculator

Sent in the query string. Premium parameters are accepted on every plan but only take effect on plans that include them. Anything not listed here is dropped rather than passed through.

Required

Optional

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curl "https://api.apiverve.com/v1/ovulationcalculator?last_period=2024-01-01&cycle_length=28" \
  -H "x-api-key: YOUR_API_KEY"

Authentication

Send your key in the x-api-key header. That is the only auth step — no token exchange, and no per-endpoint scope to configure.

Response — GET /v1/ovulationcalculator

Every APIVerve endpoint returns the same three top-level keys, so one response handler covers your whole integration: status, error and data. Only data changes shape.

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{
  "status": "ok",
  "error": null,
  "data": {
    "last_period": "2024-01-01",
    "cycle_length": 28,
    "ovulation": {
      "date": "2024-01-15",
      "days_from_last_period": 14
    },
    "fertile_window": {
      "start": "2024-01-10",
      "end": "2024-01-15",
      "duration_days": 6
    },
    "most_fertile_days": {
      "start": "2024-01-13",
      "end": "2024-01-15",
      "duration_days": 3
    },
    "fertile_days": [
      {
        "date": "2024-01-10",
        "day_relative_to_ovulation": -5,
        "fertility_level": "medium",
        "description": "Fertile"
      },
      {
        "date": "2024-01-11",
        "day_relative_to_ovulation": -4,
        "fertility_level": "medium",
        "description": "Fertile"
      },
      {
        "date": "2024-01-12",
        "day_relative_to_ovulation": -3,
        "fertility_level": "medium",
        "description": "Fertile"
      },
      {
        "date": "2024-01-13",
        "day_relative_to_ovulation": -2,
        "fertility_level": "high",
        "description": "Most fertile"
      },
      {
        "date": "2024-01-14",
        "day_relative_to_ovulation": -1,
        "fertility_level": "high",
        "description": "Most fertile"
      },
      {
        "date": "2024-01-15",
        "day_relative_to_ovulation": 0,
        "fertility_level": "high",
        "description": "Most fertile"
      }
    ],
    "next_period": {
      "date": "2024-01-29",
      "days_from_last_period": 28
    },
    "cycle_phases": {
      "follicular_phase": {
        "duration_days": 14,
        "description": "From first day of period to ovulation"
      },
      "ovulation": {
        "duration_days": 1,
        "description": "Egg is released from ovary"
      },
      "luteal_phase": {
        "duration_days": 14,
        "description": "From ovulation to next period"
      }
    },
    "current_status": {
      "current_phase": "Menstruation",
      "is_fertile": false,
      "days_until_ovulation": -701,
      "days_until_next_period": -687
    },
    "disclaimer": "This calculator provides estimates only. Actual ovulation may vary. Consult a healthcare provider for medical advice."
  }
}

Response fields

Paths are relative to data. Premium fields are absent rather than zeroed on plans that do not include them, so check for presence instead of comparing to 0.

Response headers

Every response carries your balance, so your own code always knows where it stands without polling anything. All four are exposed to browsers through CORS.

Errors

Read the HTTP status first, then error for the specific reason. The body names the parameter that has to change.

What a call costs — 1 credits per call

Credits are shared across every APIVerve API — one balance, one key, one bill. Failed requests never count against it, so a rejected input costs nothing.

Calls are what the credits buy at this API's rate — spend them here, on any of the other APIs, or across both. Nothing is reserved per endpoint.

Zero-code embed — one snippet, no backend

Drop an interactive Ovulation Calculator form onto any page. The widget calls the API for you, so no key ever appears in your markup and there is nothing to deploy.

Changelog — Ovulation Calculator API

Every change to this endpoint, newest first. Breaking changes ship as a new version and the previous one keeps serving — response fields are added, never removed or retyped in place, so an integration written against v1 keeps working.

Currently v1