Menstrual Cycle Calculator API Reference

Overview

Provides current cycle status, PMS timing, and comprehensive period tracking information, perfect for women's health apps and cycle monitoring applications.

Endpoint

One host, one path per API. The block below shows this call in four languages; every one of them is the same HTTP request. Making requests covers the timeouts, retries and parameter rules that apply to all of them. The SDKs wrap the same call in a typed client.

GEThttps://api.apiverve.com/v1/menstrualcycle

curl "https://api.apiverve.com/v1/menstrualcycle?last_period=2024-01-01&cycle_length=28&period_length=5&cycles=3" \
  -H "x-api-key: your_api_key_here"

const res = await fetch('https://api.apiverve.com/v1/menstrualcycle?last_period=2024-01-01&cycle_length=28&period_length=5&cycles=3', {
  headers: { 'x-api-key': 'your_api_key_here' },
});

if (!res.ok) throw new Error(`${res.status} ${await res.text()}`);

const { data } = await res.json();
console.log(data);

import requests

res = requests.get(
    "https://api.apiverve.com/v1/menstrualcycle?last_period=2024-01-01&cycle_length=28&period_length=5&cycles=3",
    headers={"x-api-key": "your_api_key_here"},
    timeout=15,
)
res.raise_for_status()

print(res.json()["data"])

package main

import (
	"fmt"
	"io"
	"net/http"
)

func main() {
	req, _ := http.NewRequest("GET", "https://api.apiverve.com/v1/menstrualcycle?last_period=2024-01-01&cycle_length=28&period_length=5&cycles=3", nil)
	req.Header.Set("x-api-key", "your_api_key_here")

	res, err := http.DefaultClient.Do(req)
	if err != nil {
		panic(err)
	}
	defer res.Body.Close()

	out, _ := io.ReadAll(res.Body)
	fmt.Println(string(out))
}

Replace your_api_key_here with the key from your dashboard. When the inputs arrive as a list rather than one at a time, batch requests run up to 200 of them through this same API in a single call.

Authentication

Send your key in the x-api-key header. That is the only auth step — there is no token exchange and no per-endpoint scope to configure. Authentication covers creating, rotating and revoking keys.

401 is the only auth verdict

A 401 means the key is missing, invalid or expired. A 403 means the key is valid but not permitted here — blocked by a key restriction or an IP allow-list. Running out of credits is a 429.

Parameters

Sent in the query string. Premium parameters are accepted on every plan but only take effect on plans that include them.

Parameter	Type	Description
`last_period`Required	string	First day of last menstrual period (YYYY-MM-DD) date
`cycle_length`Optional	integer	Average cycle length in days default 28 · range 21–35
`period_length`Optional	integer	Average period duration in days default 5 · range 2–10
`cycles`Optional	integer	Number of future cycles to calculate default 3 · range 1–12

Response

Every API returns the same three top-level keys, so one response handler covers your whole integration: status, error and data. Only data changes shape. Response format covers the envelope, the other output formats and how premium fields are withheld.

Sample response

{
  "status": "ok",
  "error": null,
  "data": {
    "last_period_date": "2024-01-01",
    "cycle_length": 28,
    "period_length": 5,
    "cycles_calculated": 3,
    "cycles": [
      {
        "cycle_number": 1,
        "period": {
          "start_date": "2024-01-01",
          "end_date": "2024-01-05",
          "duration_days": 5
        },
        "ovulation": {
          "date": "2024-01-15",
          "day_of_cycle": 14
        },
        "fertile_window": {
          "start_date": "2024-01-10",
          "end_date": "2024-01-15",
          "duration_days": 6
        },
        "pms_phase": {
          "start_date": "2023-12-21",
          "end_date": "2023-12-31"
        },
        "cycle_phases": {
          "menstrual": {
            "start": "2024-01-01",
            "end": "2024-01-05",
            "description": "Menstruation"
          },
          "follicular": {
            "start": "2024-01-06",
            "end": "2024-01-14",
            "description": "Follicular phase - preparing for ovulation"
          },
          "ovulation": {
            "date": "2024-01-15",
            "description": "Ovulation - most fertile day"
          },
          "luteal": {
            "start": "2024-01-16",
            "end": "2024-01-28",
            "description": "Luteal phase - preparing for next period"
          }
        },
        "status": "past",
        "days_until": null,
        "days_ago": 715
      },
      {
        "cycle_number": 2,
        "period": {
          "start_date": "2024-01-29",
          "end_date": "2024-02-02",
          "duration_days": 5
        },
        "ovulation": {
          "date": "2024-02-12",
          "day_of_cycle": 14
        },
        "fertile_window": {
          "start_date": "2024-02-07",
          "end_date": "2024-02-12",
          "duration_days": 6
        },
        "pms_phase": {
          "start_date": "2024-01-18",
          "end_date": "2024-01-28"
        },
        "cycle_phases": {
          "menstrual": {
            "start": "2024-01-29",
            "end": "2024-02-02",
            "description": "Menstruation"
          },
          "follicular": {
            "start": "2024-02-03",
            "end": "2024-02-11",
            "description": "Follicular phase - preparing for ovulation"
          },
          "ovulation": {
            "date": "2024-02-12",
            "description": "Ovulation - most fertile day"
          },
          "luteal": {
            "start": "2024-02-13",
            "end": "2024-02-25",
            "description": "Luteal phase - preparing for next period"
          }
        },
        "status": "past",
        "days_until": null,
        "days_ago": 687
      },
      {
        "cycle_number": 3,
        "period": {
          "start_date": "2024-02-26",
          "end_date": "2024-03-01",
          "duration_days": 5
        },
        "ovulation": {
          "date": "2024-03-11",
          "day_of_cycle": 14
        },
        "fertile_window": {
          "start_date": "2024-03-06",
          "end_date": "2024-03-11",
          "duration_days": 6
        },
        "pms_phase": {
          "start_date": "2024-02-15",
          "end_date": "2024-02-25"
        },
        "cycle_phases": {
          "menstrual": {
            "start": "2024-02-26",
            "end": "2024-03-01",
            "description": "Menstruation"
          },
          "follicular": {
            "start": "2024-03-02",
            "end": "2024-03-10",
            "description": "Follicular phase - preparing for ovulation"
          },
          "ovulation": {
            "date": "2024-03-11",
            "description": "Ovulation - most fertile day"
          },
          "luteal": {
            "start": "2024-03-12",
            "end": "2024-03-24",
            "description": "Luteal phase - preparing for next period"
          }
        },
        "status": "past",
        "days_until": null,
        "days_ago": 659
      }
    ],
    "current_status": {
      "phase": "Unknown",
      "description": "",
      "next_period": null,
      "days_until_next_period": null
    },
    "averages": {
      "cycle_length": 28,
      "period_duration": 5,
      "days_between_periods": 28
    },
    "disclaimer": "This calculator provides estimates only. Actual cycles 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.

Field	Type	Example	Description
`last_period_date`	string	`"2024-01-01"`	First day of last menstrual period
`cycle_length`	number	`28`	Average menstrual cycle length in days
`period_length`	number	`5`	Average menstrual period duration in days
`cycles_calculated`	number	`3`	Total number of cycles calculated
`cycles`	array[3]		Array of calculated menstrual cycles with details
`cycle_number`	number	`1`	Sequential number of the cycle
`period`	object	`{...}`	Start, end and length of the period for this cycle
`start_date`	string	`"2024-01-01"`	Start date of menstrual period
`end_date`	string	`"2024-01-05"`	End date of menstrual period
`duration_days`	number	`5`	Duration of period in days
`ovulation`	object	`{...}`	Estimated ovulation date and which day of the cycle it falls on
`date`	string	`"2024-01-15"`	Predicted ovulation date in cycle
`day_of_cycle`	number	`14`	Day of cycle when ovulation occurs
`fertile_window`	object	`{...}`	Range of days conception is possible in this cycle
`start_date`	string	`"2024-01-10"`	Start of fertile window for conception
`end_date`	string	`"2024-01-15"`	End of fertile window for conception
`duration_days`	number	`6`	Duration of fertile window in days
`pms_phase`	object	`{...}`	Range of days premenstrual symptoms are expected
`start_date`Premium	string	`"2023-12-21"`	Start date of PMS phase
`end_date`Premium	string	`"2023-12-31"`	End date of PMS phase
`cycle_phases`Premium	object	`{...}`	Detailed breakdown of menstrual cycle phases
`menstrual`Premium	object	`{...}`	The bleeding phase, with its dates
`start`Premium	string	`"2024-01-01"`	Start date of menstrual phase
`end`Premium	string	`"2024-01-05"`	End date of menstrual phase
`description`Premium	string	`"Menstruation"`	Description of menstrual phase
`follicular`Premium	object	`{...}`	From the end of the period to ovulation
`start`Premium	string	`"2024-01-06"`	Start date of follicular phase
`end`Premium	string	`"2024-01-14"`	End date of follicular phase
`description`Premium	string	`"Follicular phase - preparing for ovulation"`	Description of follicular phase
`ovulation`Premium	object	`{...}`	The day the egg is released
`date`Premium	string	`"2024-01-15"`	Ovulation date within cycle
`description`Premium	string	`"Ovulation - most fertile day"`	Description of ovulation phase
`luteal`Premium	object	`{...}`	From ovulation to the start of the next period
`start`Premium	string	`"2024-01-16"`	Start date of luteal phase
`end`Premium	string	`"2024-01-28"`	End date of luteal phase
`description`Premium	string	`"Luteal phase - preparing for next period"`	Description of luteal phase
`status`Premium	string	`"past"`	Status of cycle relative to current date
`days_until`Premium	object	`null`	Days until cycle phase occurs
`days_ago`Premium	number	`715`	Days since cycle phase occurred
`current_status`	object	`{...}`	Which phase today falls in and when the next period is due
`phase`Premium	string	`"Unknown"`	Current menstrual cycle phase name
`description`Premium	string	`""`	Description of current cycle phase
`next_period`Premium	object	`null`	Date of next expected menstrual period
`days_until_next_period`Premium	object	`null`	Days until next menstrual period starts
`averages`	object	`{...}`	Average cycle length, period duration and gap between periods
`cycle_length`Premium	number	`28`	Average menstrual cycle length across periods
`period_duration`Premium	number	`5`	Average menstrual period duration across cycles
`days_between_periods`Premium	number	`28`	Average days between menstrual period starts
`disclaimer`	string	`"This calculator provides estimates only. Actual cycles may vary. Consult a healthcare provider for medical advice."`	Legal disclaimer about calculation accuracy

Errors

Read the HTTP status first, then error for the specific reason. The body names the parameter that has to change. Error handling covers the full status list and which of them are worth retrying.

Status	Meaning	What to do
`400`	Input was rejected	Read `error`; it names the parameter.
`401`	Key missing or invalid	Check the header name and the key value.
`403`	Key valid, but not permitted	A key restriction or IP allow-list; see key scoping.
`429`	Rate limited, or out of credits	Read `error` to tell them apart; see rate limits.

Use cases

Period Tracking: Build period tracking applications with accurate cycle predictions, period start dates, and phase identification
Cycle Prediction: Predict future menstrual cycles with customizable cycle length and period duration for planning and preparation
Health Monitoring: Create women's health monitoring tools with detailed cycle phase information including follicular, ovulation, and luteal phases
Health Insights: Provide cycle-based health insights and symptom tracking aligned with menstrual phases for better health awareness

Other ways to use Menstrual Cycle Calculator

Set up Menstrual Cycle Calculator on APIVerve, or reach the same source a different way. Your APIVerve account and credits work on all of them — one key, one balance.

Give it to an AI agentConnect over MCP and your agent calls it as a native tool — Claude, Cursor, ChatGPT.VerveKit →Reference →

Use it in Google Sheets or ExcelA =VERVE() formula fills a column — no script, no export, recalculates in place.VerveSheets →Reference →

Ground an agent on itA cited, machine-checkable fact your model can't produce on its own.VerveContext →Reference →

Menstrual Cycle Calculator