Pregnancy Due Date & Conception Calculator
Use this calculator to estimate your due date, conception date, and current gestational age based on the first day of your last menstrual period (LMP).
Understanding Your Pregnancy Dates
Calculating your due date is one of the first and most exciting steps after discovering you're pregnant. While an ultrasound provides the most accurate dating, a period calculator offers a good initial estimate based on your last menstrual period (LMP).
How the Calculator Works
This calculator primarily uses Naegele's Rule, a standard method for estimating a due date. It works on the assumption that pregnancy lasts approximately 280 days (40 weeks) from the first day of your last menstrual period. It also assumes a typical 28-day menstrual cycle with ovulation occurring around day 14.
- Estimated Due Date (EDD): This is calculated by adding 280 days (40 weeks) to the first day of your LMP. This date is when your baby is considered full-term.
- Estimated Conception Date: This date is approximated by adding your average cycle length minus 14 days to your LMP. For a standard 28-day cycle, this means adding 14 days to your LMP, as ovulation typically occurs around day 14.
- Current Gestational Age: This tells you how many weeks and days pregnant you are right now, based on your LMP and today's date.
Why Your LMP is Important
The first day of your last menstrual period is crucial because it's often the most reliable fixed point in time that women remember. Even though conception happens later, doctors use the LMP as the starting point for pregnancy dating because ovulation and conception dates can be harder to pinpoint precisely.
Factors That Can Affect Accuracy
While helpful, period calculators have limitations:
- Irregular Cycles: If your menstrual cycles are irregular, longer, or shorter than the average 28 days, the calculator's estimates for conception and due date may be less accurate.
- Uncertain LMP: If you can't remember your LMP or had recent hormonal birth control use, the calculation will be less reliable.
- Early Ultrasound: An early ultrasound (typically between 8 and 12 weeks) is considered the most accurate way to date a pregnancy, especially if there's a discrepancy with LMP-based dating.
What to Do with Your Estimated Dates
These dates provide a good starting point for your pregnancy journey. Share them with your healthcare provider, who will confirm your due date through clinical examination and potentially an ultrasound. Remember, only about 5% of babies are born exactly on their due date; it's more of a target window!
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function calculatePregnancyDates() {
var lmpDateInput = document.getElementById("lastMenstrualPeriodDate").value;
var cycleLengthInput = document.getElementById("averageCycleLength").value;
var errorMessageDiv = document.getElementById("errorMessage");
errorMessageDiv.innerHTML = ""; // Clear previous errors
if (!lmpDateInput) {
errorMessageDiv.innerHTML = "Please enter the first day of your Last Menstrual Period.";
document.getElementById("resultDueDate").innerHTML = "";
document.getElementById("resultConceptionDate").innerHTML = "";
document.getElementById("resultGestationalAge").innerHTML = "";
return;
}
var lmpDate = new Date(lmpDateInput);
if (isNaN(lmpDate.getTime())) {
errorMessageDiv.innerHTML = "Invalid LMP date entered. Please use a valid date format.";
document.getElementById("resultDueDate").innerHTML = "";
document.getElementById("resultConceptionDate").innerHTML = "";
document.getElementById("resultGestationalAge").innerHTML = "";
return;
}
var cycleLength = parseInt(cycleLengthInput);
if (isNaN(cycleLength) || cycleLength 45) {
errorMessageDiv.innerHTML = "Please enter a valid average cycle length (between 20 and 45 days).";
document.getElementById("resultDueDate").innerHTML = "";
document.getElementById("resultConceptionDate").innerHTML = "";
document.getElementById("resultGestationalAge").innerHTML = "";
return;
}
// Calculate Estimated Due Date (EDD) – Naegele's Rule: LMP + 280 days (40 weeks)
var edd = new Date(lmpDate.getTime());
edd.setDate(edd.getDate() + 280);
// Calculate Estimated Conception Date: LMP + (cycleLength – 14) days
// Assuming ovulation is 14 days before the *next* period, so (cycleLength – 14) days after LMP
var conceptionDate = new Date(lmpDate.getTime());
conceptionDate.setDate(conceptionDate.getDate() + (cycleLength – 14));
// Calculate Current Gestational Age
var today = new Date();
// Normalize today's date to midnight for accurate day calculation
today.setHours(0, 0, 0, 0);
lmpDate.setHours(0, 0, 0, 0); // Normalize LMP date too
var timeDiff = today.getTime() – lmpDate.getTime();
var daysPregnant = Math.floor(timeDiff / (1000 * 60 * 60 * 24));
var weeksPregnant = Math.floor(daysPregnant / 7);
var remainingDays = daysPregnant % 7;
// Format dates for display
var options = { year: 'numeric', month: 'long', day: 'numeric' };
var formattedEdd = edd.toLocaleDateString('en-US', options);
var formattedConceptionDate = conceptionDate.toLocaleDateString('en-US', options);
// Display results
document.getElementById("resultDueDate").innerHTML = "
Estimated Due Date: " + formattedEdd;
document.getElementById("resultConceptionDate").innerHTML = "
Estimated Conception Date: " + formattedConceptionDate;
if (daysPregnant >= 0) {
document.getElementById("resultGestationalAge").innerHTML = "
Current Gestational Age: " + weeksPregnant + " weeks and " + remainingDays + " days";
} else {
document.getElementById("resultGestationalAge").innerHTML = "
Current Gestational Age: Not yet pregnant based on LMP.";
}
}