function calculatePregnancy() {
var lmpInput = document.getElementById("lmpDate").value;
var cycleInput = parseInt(document.getElementById("cycleLength").value);
if (!lmpInput) {
alert("Please select the first day of your last period.");
return;
}
var lmpDate = new Date(lmpInput);
var today = new Date();
today.setHours(0, 0, 0, 0);
// Cycle adjustment: Most due date calculations assume a 28 day cycle.
// We adjust the LMP based on the difference from 28 days.
var cycleAdjustment = cycleInput – 28;
// Due Date Calculation (Naegele's Rule is LMP + 280 days)
var dueDate = new Date(lmpDate);
dueDate.setDate(dueDate.getDate() + 280 + cycleAdjustment);
// Estimated Conception (Usually LMP + 14 days for a 28 day cycle)
var conceptionDate = new Date(lmpDate);
conceptionDate.setDate(conceptionDate.getDate() + 14 + cycleAdjustment);
// Current Progress (Difference between today and adjusted LMP)
var diffInMs = today.getTime() – lmpDate.getTime();
var diffInDays = Math.floor(diffInMs / (1000 * 60 * 60 * 24));
var currentWeeks = Math.floor(diffInDays / 7);
var currentDays = diffInDays % 7;
// Days remaining
var msRemaining = dueDate.getTime() – today.getTime();
var daysRemaining = Math.ceil(msRemaining / (1000 * 60 * 60 * 24));
// Trimester logic
var trimester = "";
if (currentWeeks < 13) {
trimester = "First Trimester";
} else if (currentWeeks < 27) {
trimester = "Second Trimester";
} else {
trimester = "Third Trimester";
}
// Display formatting
var options = { month: 'long', day: 'numeric', year: 'numeric' };
document.getElementById("resDueDate").innerText = dueDate.toLocaleDateString(undefined, options);
document.getElementById("resConception").innerText = conceptionDate.toLocaleDateString(undefined, options);
if (diffInDays < 0) {
document.getElementById("resCurrentWeek").innerText = "Not yet started";
document.getElementById("resTrimester").innerText = "N/A";
document.getElementById("resDaysLeft").innerText = "Your pregnancy journey hasn't reached day one relative to this LMP yet.";
} else if (daysRemaining < 0) {
document.getElementById("resCurrentWeek").innerText = "Post-term";
document.getElementById("resTrimester").innerText = "Third Trimester";
document.getElementById("resDaysLeft").innerText = "You have passed your estimated due date!";
} else {
document.getElementById("resCurrentWeek").innerText = currentWeeks + " Weeks, " + currentDays + " Days";
document.getElementById("resTrimester").innerText = trimester;
document.getElementById("resDaysLeft").innerText = "You have approximately " + daysRemaining + " days to go!";
}
document.getElementById("pregnancy-results").style.display = "block";
}
How the Pregnancy Due Date is Calculated
Most healthcare providers estimate a baby's due date based on the date of your Last Menstrual Period (LMP). This is because most women don't know exactly when they ovulated or conceived, but they do know when their last period started.
Naegele's Rule
Our calculator primarily uses Naegele's Rule, which is the standard method for calculating due dates. It assumes a human pregnancy lasts roughly 280 days (40 weeks) from the first day of the last menstrual period. The math follows a simple pattern: take the first day of your last period, add one year, subtract three months, and add seven days.
Adjusting for Cycle Length
Not every woman has a perfect 28-day cycle. If your cycle is consistently longer or shorter, your ovulation date—and therefore your conception date—will shift. Our calculator allows you to input your average cycle length to provide a more accurate estimation of your due date.
The Three Trimesters
First Trimester (Week 1–Week 12): A time of rapid development where the baby's body structure and organ systems form.
Second Trimester (Week 13–Week 26): Often called the "golden period," many women feel an increase in energy. The baby begins to move and hear.
Third Trimester (Week 27–End): The final stretch where the baby grows significantly in size and weight, preparing for life outside the womb.
Is the Due Date a Guarantee?
It is important to remember that a due date is just an estimate. Research shows that only about 4% to 5% of babies are born exactly on their estimated due date. Most babies arrive between 37 and 42 weeks of pregnancy. This tool is designed for informational purposes and should not replace the advice of a medical professional.
Example Calculation
If your last period started on January 1st and you have a standard 28-day cycle:
Add 280 days to January 1st.
Your estimated due date would be October 8th.
If you are checking this on March 1st, you would be 8 weeks and 4 days pregnant.