Enter the first day of your last menstrual period (LMP) to estimate your baby's due date.
Understanding Your Estimated Due Date (EDD)
Calculating your baby's estimated due date (EDD) is a crucial step in tracking your pregnancy journey. While it's an estimate and only a small percentage of babies are born exactly on their due date, it provides a vital timeframe for monitoring your baby's growth and development, and for preparing for the arrival of your little one.
How the Due Date Calculator Works (Naegele's Rule)
The most common method for calculating an estimated due date is known as Naegele's Rule. This rule is based on the assumption that a typical pregnancy lasts 40 weeks (280 days) from the first day of the last menstrual period (LMP).
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 simpler to implement computationally:
Step 1: Take the first day of your last menstrual period (LMP).
Step 2: Add 40 weeks (280 days) to this date.
Our calculator uses the second, more direct method of adding 280 days to your LMP for accuracy and simplicity.
Important Considerations:
It's important to remember that the EDD is an estimate. Factors such as the regularity of your menstrual cycles before conception, ovulation timing, and individual variations in pregnancy length can affect the actual birth date.
Irregular Cycles: If your menstrual cycles are irregular, the LMP date might not accurately reflect the start of your pregnancy. In such cases, your doctor or midwife may use early ultrasound scans to provide a more accurate EDD.
Ovulation: Naegele's Rule assumes ovulation occurs around day 14 of a 28-day cycle. If you ovulated earlier or later, your actual due date could vary.
Ultrasound Dating: Early pregnancy ultrasounds (especially those done in the first trimester) are considered the most accurate method for dating a pregnancy if there's uncertainty about your LMP or if your cycles are irregular.
Why is the Due Date Important?
Your EDD is a valuable tool for:
Monitoring Fetal Development: Healthcare providers use it to track your baby's growth milestones.
Scheduling Appointments: Prenatal check-ups are often scheduled based on your pregnancy stage relative to your EDD.
Planning: It helps you and your partner prepare for the baby's arrival, including packing the hospital bag and arranging parental leave.
Medical Decisions: In some situations, medical interventions may be considered if a pregnancy goes significantly past the due date.
Always discuss your due date and any concerns with your healthcare provider. This calculator is a helpful tool but should not replace professional medical advice.
function calculateDueDate() {
var lmpInput = document.getElementById("lastPeriodStart");
var resultDiv = document.getElementById("result");
var lmpValue = lmpInput.value;
if (!lmpValue) {
resultDiv.innerHTML = "Please enter the first day of your last menstrual period.";
resultDiv.style.backgroundColor = "#f8d7da";
resultDiv.style.color = "#721c24";
resultDiv.style.borderColor = "#f5c6cb";
return;
}
try {
var lmpDate = new Date(lmpValue);
// Check if the date is valid
if (isNaN(lmpDate.getTime())) {
throw new Error("Invalid date format");
}
// Add 280 days (40 weeks) to the LMP date
var edd = new Date(lmpDate.getTime());
edd.setDate(lmpDate.getDate() + 280);
// Format the date for display
var options = { year: 'numeric', month: 'long', day: 'numeric' };
var formattedEdd = edd.toLocaleDateString(undefined, options);
resultDiv.innerHTML = "Your Estimated Due Date is: " + formattedEdd;
resultDiv.style.backgroundColor = "#d4edda";
resultDiv.style.color = "#155724";
resultDiv.style.borderColor = "#c3e6cb";
} catch (error) {
resultDiv.innerHTML = "Error calculating due date. Please ensure the date is entered correctly.";
resultDiv.style.backgroundColor = "#f8d7da";
resultDiv.style.color = "#721c24";
resultDiv.style.borderColor = "#f5c6cb";
console.error("Date calculation error:", error);
}
}