Accurate Menstrual Calculator

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Accurate Menstrual Cycle & Ovulation Calculator

Usually 21-35 days

Your Cycle Predictions

Next Period Starts:

Estimated Ovulation Day:

Fertile Window:

Due Date (If Conceived):

function calculateMenstrualData() { var lastDateInput = document.getElementById('lastPeriodDate').value; var cycleLength = parseInt(document.getElementById('avgCycleLength').value); var periodLen = parseInt(document.getElementById('avgPeriodDuration').value); if (!lastDateInput) { alert("Please select the start date of your last period."); return; } var lastDate = new Date(lastDateInput); if (isNaN(lastDate.getTime())) { alert("Invalid date format."); return; } // Calculation Logic // 1. Next Period var nextPeriod = new Date(lastDate); nextPeriod.setDate(lastDate.getDate() + cycleLength); // 2. Ovulation (typically 14 days before next period) var ovulationDate = new Date(nextPeriod); ovulationDate.setDate(nextPeriod.getDate() – 14); // 3. Fertile Window (5 days before ovulation + ovulation day) var fertileStart = new Date(ovulationDate); fertileStart.setDate(ovulationDate.getDate() – 5); var fertileEnd = new Date(ovulationDate); fertileEnd.setDate(ovulationDate.getDate() + 1); // 4. Due Date (Naegele's Rule: Last period + 280 days) var dueDate = new Date(lastDate); dueDate.setDate(lastDate.getDate() + 280); // Format Dates var options = { weekday: 'short', year: 'numeric', month: 'short', day: 'numeric' }; document.getElementById('nextPeriodOutput').innerText = nextPeriod.toLocaleDateString('en-US', options); document.getElementById('ovulationOutput').innerText = ovulationDate.toLocaleDateString('en-US', options); document.getElementById('fertileOutput').innerText = fertileStart.toLocaleDateString('en-US', {month: 'short', day: 'numeric'}) + " – " + fertileEnd.toLocaleDateString('en-US', {month: 'short', day: 'numeric'}); document.getElementById('dueDateOutput').innerText = dueDate.toLocaleDateString('en-US', options); document.getElementById('menstrualResult').style.display = 'block'; }

Understanding Your Menstrual Cycle

A menstrual cycle is measured from the first day of one period to the first day of the next. While the average cycle is 28 days, it is perfectly normal for cycles to range from 21 to 35 days in adults and 21 to 45 days in young teens.

How This Accurate Menstrual Calculator Works

This tool uses biological algorithms to project your future dates. To get the most accurate results, you need to know your average cycle length. If you aren't sure, track your period for three months and calculate the average number of days between the start of each period.

  • Menstrual Phase: The days you are bleeding.
  • Follicular Phase: The time between the first day of the period and ovulation. Estrogen rises as an egg prepares to be released.
  • Ovulation: The release of the egg from the ovary. This is your peak fertile time.
  • Luteal Phase: The time between ovulation and the start of your next period.

Tracking Ovulation and Fertility

The "Fertile Window" represents the days in your cycle when pregnancy is most likely. Because sperm can live inside the female body for up to 5 days, and the egg lives for 12-24 hours after release, the total fertile window is roughly 6 days long.

Example Calculation

If your last period started on January 1st and you have a 28-day cycle:

  • Next Period: January 29th
  • Ovulation: January 15th
  • Fertile Window: January 10th – January 16th

Why Accuracy Varies

External factors such as stress, illness, significant weight changes, and travel can impact the timing of your cycle. This calculator is a projection based on mathematical averages. For medical concerns or family planning, it is recommended to combine this tool with basal body temperature (BBT) tracking or ovulation predictor kits (OPKs).

Disclaimer: This calculator is for informational purposes only and should not be used as a primary method of birth control or for medical diagnosis. Consult with a healthcare professional for personalized advice.

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