Calculating your estimated due date (EDD) is a crucial part of pregnancy. While it's an estimate and only a small percentage of babies are born exactly on their due date, it provides a valuable roadmap for monitoring your pregnancy's progress. This calculator uses the Naegele's Rule, a common method that bases the EDD on the first day of your last menstrual period (LMP).
How the Calculator Works (Naegele's Rule)
The standard pregnancy term is considered to be 40 weeks (280 days) from the first day of your last menstrual period. Naegele's Rule is a simple formula for estimating this:
Step 1: Take the first day of your Last Menstrual Period (LMP).
Step 2: Add 7 days to this date.
Step 3: Subtract 3 months from the resulting date.
Step 4: Add 1 year to the resulting date.
Alternatively, and often more straightforward for calculators, is to add 280 days (40 weeks) to the LMP date.
Calculating Current Gestational Age and Remaining Time
This calculator also helps you understand your current stage of pregnancy and how much time is left until your estimated due date.
It determines the number of weeks and days that have passed since your LMP.
It calculates the number of days and weeks remaining until your estimated due date.
Important Considerations
Irregular Cycles: Naegele's Rule assumes a regular 28-day cycle with ovulation occurring around day 14. If your cycles are significantly longer or shorter, or if ovulation occurred earlier or later than expected, your actual due date may differ.
Early Pregnancy Confirmation: The most accurate way to confirm your due date is through an early ultrasound, particularly one performed in the first trimester (typically between 8-12 weeks of gestation). This can provide a more precise gestational age.
What "Due Date" Means: Your due date is an estimate. A full-term pregnancy is generally considered to be between 37 and 42 weeks. Babies born within this window are considered full-term.
Consult Your Healthcare Provider: This calculator is for informational purposes only and should not replace professional medical advice. Always discuss your pregnancy timeline and concerns with your doctor or midwife.
function calculateDueDate() {
var lmpDateInput = document.getElementById("lmpDate").value;
var gestationalAgeWeeksInput = document.getElementById("gestationalAgeWeeks").value;
var gestationalAgeDaysInput = document.getElementById("gestationalAgeDays").value;
var resultDisplay = document.getElementById("calculatedDueDate");
var weeksIntoPregnancyDisplay = document.getElementById("weeksIntoPregnancy");
var remainingUntilDueDisplay = document.getElementById("remainingUntilDue");
// Clear previous results
resultDisplay.textContent = "–";
weeksIntoPregnancyDisplay.textContent = "–";
remainingUntilDueDisplay.textContent = "–";
if (!lmpDateInput) {
alert("Please enter the First Day of your Last Menstrual Period (LMP).");
return;
}
var lmpDate = new Date(lmpDateInput);
var today = new Date();
today.setHours(0, 0, 0, 0); // Normalize today's date to midnight
// — Calculate Due Date based on LMP —
var edd = new Date(lmpDate);
edd.setDate(edd.getDate() + 280); // Add 280 days (40 weeks)
var eddFormatted = edd.toLocaleDateString('en-US', { year: 'numeric', month: 'long', day: 'numeric' });
resultDisplay.textContent = "Estimated Due Date: " + eddFormatted;
// — Calculate Current Gestational Age and Remaining Time —
var currentGestationalAgeDays;
if (gestationalAgeWeeksInput && gestationalAgeDaysInput) {
var weeks = parseInt(gestationalAgeWeeksInput, 10);
var days = parseInt(gestationalAgeDaysInput, 10);
if (isNaN(weeks) || isNaN(days)) {
alert("Please enter valid numbers for gestational age.");
return;
}
currentGestationalAgeDays = (weeks * 7) + days;
} else {
// If weeks/days inputs are empty, calculate from LMP to today
var timeDiff = today.getTime() – lmpDate.getTime();
var diffDays = Math.floor(timeDiff / (1000 * 3600 * 24));
// Adjust if LMP is in the future (shouldn't happen with date picker, but good practice)
if (diffDays < 0) diffDays = 0;
currentGestationalAgeDays = diffDays;
}
var currentWeeks = Math.floor(currentGestationalAgeDays / 7);
var currentDays = currentGestationalAgeDays % 7;
weeksIntoPregnancyDisplay.textContent = "Current Gestational Age: " + currentWeeks + " weeks and " + currentDays + " days";
var remainingDays = 280 – currentGestationalAgeDays;
if (remainingDays < 0) remainingDays = 0; // Ensure remaining days isn't negative
var remainingWeeks = Math.floor(remainingDays / 7);
var remainingExtraDays = remainingDays % 7;
remainingUntilDueDisplay.textContent = "Days Remaining Until Due Date: " + remainingDays + " (" + remainingWeeks + " weeks and " + remainingExtraDays + " days)";
}