Understanding your pregnancy timeline is essential for planning prenatal care, lifestyle changes, and preparing for your baby's arrival. While only about 4% of babies are born on their exact due date, having an "Estimated Date of Delivery" (EDD) helps healthcare providers track fetal development milestones.
The Science: Naegele's Rule
Most online due date calculators use Naegele's Rule. This standard mathematical formula estimates the due date by adding 280 days (40 weeks) to the first day of your last menstrual period (LMP). Our calculator goes a step further by adjusting for your specific cycle length. If your cycle is longer or shorter than the standard 28 days, the calculation shifts accordingly to ensure higher accuracy.
Realistic Calculation Examples
Standard Cycle: If your LMP was January 1st and you have a 28-day cycle, your estimated due date would be October 8th.
Longer Cycle: If your LMP was January 1st but you have a 32-day cycle, the calculator adds 4 extra days, making your due date October 12th.
Shorter Cycle: If your LMP was January 1st with a 24-day cycle, the date moves forward to October 4th.
Pregnancy Trimesters Explained
A typical pregnancy is divided into three stages:
First Trimester (Weeks 1-12): A period of rapid development where major organs form.
Second Trimester (Weeks 13-26): Often called the "golden period," where many find relief from early symptoms.
Third Trimester (Weeks 27-40): The final stretch where the baby gains weight and prepares for birth.
Note: This calculator is for informational purposes only. Always consult with a qualified healthcare professional to confirm your due date via ultrasound or clinical examination.
function calculatePregnancyDate() {
var lmpInput = document.getElementById("lmpDate").value;
var cycleLength = parseInt(document.getElementById("cycleLength").value);
var resultArea = document.getElementById("resultArea");
if (!lmpInput) {
alert("Please select the date of your last period.");
return;
}
if (isNaN(cycleLength) || cycleLength 45) {
alert("Please enter a valid cycle length (20-45 days).");
return;
}
var lmpDate = new Date(lmpInput);
var today = new Date();
// Calculation: 280 days is standard for 28 day cycle.
// Adjust by (cycleLength – 28)
var adjustmentDays = 280 + (cycleLength – 28);
var eddDate = new Date(lmpDate.getTime());
eddDate.setDate(eddDate.getDate() + adjustmentDays);
// Calculate current status
var diffInMs = today – lmpDate;
var diffInDays = Math.floor(diffInMs / (1000 * 60 * 60 * 24));
var weeks = Math.floor(diffInDays / 7);
var remainingDays = diffInDays % 7;
// Calculate countdown
var countdownMs = eddDate – today;
var countdownDays = Math.ceil(countdownMs / (1000 * 60 * 60 * 24));
// Format EDD
var options = { weekday: 'long', year: 'numeric', month: 'long', day: 'numeric' };
document.getElementById("eddResult").innerText = eddDate.toLocaleDateString(undefined, options);
// Gestational Age
if (diffInDays adjustmentDays) {
document.getElementById("progressResult").innerText = "Post-term / Born";
} else {
document.getElementById("progressResult").innerText = weeks + " Weeks, " + remainingDays + " Days";
}
// Trimester
var trimester = "";
if (weeks < 13) {
trimester = "First Trimester";
} else if (weeks 0) {
document.getElementById("countdownResult").innerText = "Only " + countdownDays + " days to go!";
} else if (countdownDays === 0) {
document.getElementById("countdownResult").innerText = "Your due date is today!";
} else {
document.getElementById("countdownResult").innerText = "Baby has arrived!";
}
resultArea.style.display = "block";
resultArea.scrollIntoView({ behavior: 'smooth' });
}