Please enter the date of your last menstrual period (LMP) and the current gestational age to calculate your estimated due date.
Understanding the Pregnancy Due Date Calculator
Pregnancy is typically measured in weeks, starting from the first day of a woman's last menstrual period (LMP).
A full-term pregnancy is considered to be 40 weeks (280 days) from the LMP.
This calculator uses established medical guidelines to estimate your due date based on your LMP or a known gestational age.
How it Works: Naegele's Rule (for LMP-based calculation)
The most common method for estimating a due date is Naegele's Rule. It's a simple formula:
Add 7 days to the first day of your LMP.
Subtract 3 months from that date.
Add 1 year to the resulting date.
Alternatively, and more simply, you can add 40 weeks (280 days) to your LMP. This calculator implements the latter by adding 280 days to your entered LMP date.
Using Gestational Age:
If you know your current gestational age (either from an early ultrasound or other medical assessment), you can use that information to pinpoint your due date.
A pregnancy is considered 40 weeks (or 280 days) from the LMP. To find the due date:
Calculate the remaining weeks and days until 40 weeks is reached.
Add these remaining weeks and days to your current date.
For example, if you are exactly 20 weeks pregnant, your due date is approximately 20 weeks from now. This calculator calculates the total duration of pregnancy (40 weeks) and subtracts your current gestational age to determine the remaining time until the due date, then adds that remaining time to the current date.
Important Note:
Due dates are estimates. Only a small percentage of babies are born exactly on their due date. It's common for babies to be born within a week or two before or after their due date. This calculator is for informational purposes and should not replace professional medical advice from your doctor or midwife.
function calculateDueDate() {
var lmpDateInput = document.getElementById("lmpDate");
var gestationalAgeWeeksInput = document.getElementById("gestationalAgeWeeks");
var gestationalAgeDaysInput = document.getElementById("gestationalAgeDays");
var resultDisplay = document.querySelector("#result .due-date");
var resultParagraph = document.querySelector("#result p:not(.due-date)");
var lmpDateValue = lmpDateInput.value;
var gestationalAgeWeeks = parseInt(gestationalAgeWeeksInput.value, 10);
var gestationalAgeDays = parseInt(gestationalAgeDaysInput.value, 10);
var today = new Date();
var dueDate;
resultParagraph.style.display = 'block'; // Ensure the instructional text is visible if no calculation is done or if there's an error
if (lmpDateValue) {
var lmp = new Date(lmpDateValue);
// Check if LMP date is valid
if (isNaN(lmp.getTime())) {
resultParagraph.textContent = "Please enter a valid date for your Last Menstrual Period.";
resultDisplay.textContent = "";
return;
}
// Calculate due date using LMP (adding 280 days)
// A standard pregnancy is 280 days (40 weeks) from LMP
dueDate = new Date(lmp.getTime());
dueDate.setDate(lmp.getDate() + 280);
// If gestational age is also provided, we might use it to refine or re-calculate.
// If LMP is provided, we prioritize that as the primary calculation method.
// However, we can also calculate remaining days if gestational age is given.
if (!isNaN(gestationalAgeWeeks) && !isNaN(gestationalAgeDays)) {
var totalGestationalDays = (gestationalAgeWeeks * 7) + gestationalAgeDays;
var remainingDays = 280 – totalGestationalDays;
if (remainingDays 280) {
resultParagraph.textContent = "The provided gestational age exceeds 40 weeks.";
resultDisplay.textContent = "";
return;
}
}
var options = { year: 'numeric', month: 'long', day: 'numeric' };
resultDisplay.textContent = dueDate.toLocaleDateString(undefined, options);
resultParagraph.style.display = 'none'; // Hide instructional text when a result is shown
} else if (!isNaN(gestationalAgeWeeks) && !isNaN(gestationalAgeDays)) {
// Calculate due date based on current date and gestational age
var totalGestationalDays = (gestationalAgeWeeks * 7) + gestationalAgeDays;
var remainingDays = 280 – totalGestationalDays;
if (totalGestationalDays > 280) {
resultParagraph.textContent = "The provided gestational age exceeds 40 weeks. Due date has already passed.";
resultDisplay.textContent = "";
return;
}
if (remainingDays < 0) { // Should be covered by above, but for safety
resultParagraph.textContent = "The provided gestational age exceeds 40 weeks.";
resultDisplay.textContent = "";
return;
}
dueDate = new Date(today.getTime());
dueDate.setDate(today.getDate() + remainingDays);
var options = { year: 'numeric', month: 'long', day: 'numeric' };
resultDisplay.textContent = dueDate.toLocaleDateString(undefined, options);
resultParagraph.style.display = 'none'; // Hide instructional text when a result is shown
} else {
resultParagraph.textContent = "Please enter the date of your last menstrual period (LMP) or provide the current gestational age.";
resultDisplay.textContent = "";
}
}