Use this calculator to estimate your baby's due date based on the first day of your last menstrual period (LMP).
Enter the start date of your last period.
Understanding Your Due Date
Your estimated due date (EDD) is a crucial milestone in your pregnancy journey. While it's often referred to as a "due date," it's more accurately an "estimated" date, as only about 5% of babies are born exactly on their due date. Most babies arrive sometime between 37 and 42 weeks of gestation.
How is the Due Date Calculated? (Naegele's Rule)
The most common method for calculating a due date is Naegele's Rule. This rule assumes a 28-day menstrual cycle and that ovulation occurs on day 14. It works by adding 280 days (40 weeks) to the first day of your last menstrual period (LMP).
LMP + 280 days = Estimated Due Date
This calculator uses Naegele's Rule to provide your estimated due date, current gestational age, and a breakdown of your trimesters.
Why is the LMP Important?
The first day of your last menstrual period is considered the beginning of your pregnancy, even though conception typically occurs about two weeks later. This is because it's often the most reliable date available to track pregnancy progression.
Gestational Age and Trimesters
Pregnancy is typically divided into three trimesters, each lasting approximately 13-14 weeks. These divisions help healthcare providers monitor fetal development and maternal health at different stages.
First Trimester: From LMP to the end of week 13 (13 weeks and 6 days). This is a period of rapid development, where all major organs begin to form.
Second Trimester: From the start of week 14 to the end of week 27 (27 weeks and 6 days). Often considered the "golden trimester," as many early pregnancy symptoms subside, and fetal movement becomes noticeable.
Third Trimester: From the start of week 28 until delivery. The baby continues to grow and mature, preparing for birth.
Factors Affecting Due Date Accuracy
While Naegele's Rule is widely used, several factors can influence its accuracy:
Irregular Menstrual Cycles: If your cycles are longer or shorter than 28 days, or if they are irregular, your actual ovulation date might differ significantly from the assumed day 14, making the LMP-based due date less accurate.
Unknown LMP: If you don't remember your LMP, or if you conceived while on birth control, other methods like early ultrasound will be used.
Early Ultrasound: An ultrasound performed in the first trimester (typically between 8 and 13 weeks) can provide a very accurate due date, sometimes even more accurate than an LMP-based calculation, especially if there's uncertainty about the LMP.
Example Calculation:
Let's say the first day of your Last Menstrual Period (LMP) was January 1, 2024.
Estimated Due Date: January 1, 2024 + 280 days = October 8, 2024.
First Trimester Ends: January 1, 2024 + 97 days (13 weeks and 6 days) = April 7, 2024.
Second Trimester Ends: January 1, 2024 + 195 days (27 weeks and 6 days) = July 14, 2024.
Third Trimester Starts: January 1, 2024 + 196 days (28 weeks) = July 15, 2024.
Remember, this calculator provides an estimate. Always consult with your healthcare provider for personalized medical advice and the most accurate due date for your pregnancy.
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function calculateDueDate() {
var lmpDateInput = document.getElementById("lmpDate").value;
var resultDiv = document.getElementById("result");
resultDiv.innerHTML = ""; // Clear previous results
if (!lmpDateInput) {
resultDiv.innerHTML = "Please enter the first day of your Last Menstrual Period.";
return;
}
var lmpDate = new Date(lmpDateInput);
// Validate if the date is a valid date object
if (isNaN(lmpDate.getTime())) {
resultDiv.innerHTML = "Invalid date entered. Please use a valid date format.";
return;
}
var today = new Date();
today.setHours(0, 0, 0, 0); // Normalize today's date to start of day
// Check if LMP is in the future
if (lmpDate > today) {
resultDiv.innerHTML = "The LMP date cannot be in the future. Please enter a valid past or present date.";
return;
}
// Calculate Estimated Due Date (EDD)
// EDD = LMP + 280 days (40 weeks)
var edd = new Date(lmpDate);
edd.setDate(lmpDate.getDate() + 280);
// Calculate Current Gestational Age
var timeDiff = today.getTime() – lmpDate.getTime();
var daysDiff = Math.floor(timeDiff / (1000 * 60 * 60 * 24));
var currentWeeks = Math.floor(daysDiff / 7);
var currentDays = daysDiff % 7;
// Calculate Trimester Dates
// Trimester 1: LMP to end of week 13 (13 weeks + 6 days)
var trimester1End = new Date(lmpDate);
trimester1End.setDate(lmpDate.getDate() + (13 * 7) + 6);
// Trimester 2: Start of week 14 to end of week 27 (27 weeks + 6 days)
var trimester2End = new Date(lmpDate);
trimester2End.setDate(lmpDate.getDate() + (27 * 7) + 6);
// Trimester 3: Start of week 28 (28 weeks)
var trimester3Start = new Date(lmpDate);
trimester3Start.setDate(lmpDate.getDate() + (28 * 7));
// Format dates for display
var options = { year: 'numeric', month: 'long', day: 'numeric' };
var formattedLMP = lmpDate.toLocaleDateString('en-US', options);
var formattedEDD = edd.toLocaleDateString('en-US', options);
var formattedTrimester1End = trimester1End.toLocaleDateString('en-US', options);
var formattedTrimester2End = trimester2End.toLocaleDateString('en-US', options);
var formattedTrimester3Start = trimester3Start.toLocaleDateString('en-US', options);
// Display results
var resultsHTML = "
Your Estimated Pregnancy Details:
";
resultsHTML += "Based on your LMP of " + formattedLMP + ":";
resultsHTML += "Estimated Due Date:" + formattedEDD + "";
if (daysDiff >= 0) { // Only show gestational age if LMP is not in the future
resultsHTML += "Current Gestational Age: " + currentWeeks + " weeks and " + currentDays + " days";
} else {
resultsHTML += "Gestational age cannot be calculated as LMP is in the future.";
}
resultsHTML += "First Trimester Ends: " + formattedTrimester1End + "";
resultsHTML += "Second Trimester Ends: " + formattedTrimester2End + "";
resultsHTML += "Third Trimester Starts: " + formattedTrimester3Start + "";
resultsHTML += "Please note: Due dates are estimates. Only about 5% of babies are born on their exact due date. Always consult your healthcare provider for personalized advice.";
resultDiv.innerHTML = resultsHTML;
}