Enter the first day of your last menstrual period (LMP) to estimate your due date.
Understanding Your Pregnancy Due Date
The pregnancy due date calculator is a vital tool for expectant parents and healthcare providers. It helps estimate when a baby is likely to be born, based on the mother's last menstrual period (LMP). While often referred to as a "due date," it's more accurately an estimated window for birth, as full-term pregnancy can range from 37 to 42 weeks.
How the Calculation Works: Naegele's Rule
The most common method for calculating a due date is Naegele's Rule. This method assumes a typical 28-day menstrual cycle where ovulation occurs around day 14. The rule is as follows:
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.
This calculation effectively adds 280 days (or 40 weeks) to the first day of your LMP.
Adjusting for Cycle Length:
Naegele's Rule is an approximation. Many women have menstrual cycles longer or shorter than 28 days. Our calculator also takes your average cycle length into account. If your cycle is longer than 28 days, your due date might be slightly later than Naegele's Rule suggests. If your cycle is shorter, it might be slightly earlier. The calculator adjusts by adding or subtracting the difference between your average cycle length and 28 days to the estimated conception date.
Important Considerations:
Accuracy: This calculator provides an estimate. Not all babies are born exactly on their due date.
Ovulation Variations: Irregular cycles or variations in ovulation can affect the accuracy of LMP-based calculations.
Early Pregnancy Confirmation: For a more precise dating, especially in cases of irregular cycles or uncertainty about the LMP, healthcare providers often use early ultrasound scans.
Consult Your Doctor: Always discuss your due date and any pregnancy-related concerns with your healthcare provider.
Use this calculator as a helpful guide to track your pregnancy journey.
function calculateDueDate() {
var lmpDateInput = document.getElementById("lmpDate");
var cycleLengthInput = document.getElementById("cycleLength");
var resultDiv = document.getElementById("result");
var lmpDateStr = lmpDateInput.value;
var cycleLength = parseInt(cycleLengthInput.value);
if (!lmpDateStr || isNaN(cycleLength) || cycleLength 40) {
resultDiv.innerHTML = "Please enter a valid LMP date and cycle length (21-40 days).";
return;
}
var lmpDate = new Date(lmpDateStr);
// Adjust for invalid date input that might pass basic validation
if (isNaN(lmpDate.getTime())) {
resultDiv.innerHTML = "Invalid date format for LMP. Please use YYYY-MM-DD.";
return;
}
// Naegele's Rule calculation: LMP + 7 days – 3 months + 1 year = 40 weeks (280 days)
// Or more simply: LMP + 280 days
var dueDate = new Date(lmpDate);
dueDate.setDate(lmpDate.getDate() + 280);
// Adjust for cycle length difference from 28 days.
// If cycleLength > 28, the baby might come later, so add the difference.
// If cycleLength < 28, the baby might come earlier, so subtract the difference.
var cycleDifference = cycleLength – 28;
dueDate.setDate(dueDate.getDate() + cycleDifference);
var options = { year: 'numeric', month: 'long', day: 'numeric' };
var formattedDueDate = dueDate.toLocaleDateString(undefined, options);
resultDiv.innerHTML = "Estimated Due Date: " + formattedDueDate + "";
}