Calculate your estimated due date based on the first day of your last menstrual period (LMP) or by providing a known pregnancy scan date.
Understanding Pregnancy Weeks and Due Dates
Calculating the estimated due date (EDD) is a crucial part of prenatal care. It helps healthcare providers monitor the progress of the pregnancy and determine appropriate interventions. The most common method relies on the Last Menstrual Period (LMP), but early ultrasounds can also provide highly accurate dating.
Method 1: Using the Last Menstrual Period (LMP)
This method, often called Naegele's Rule, assumes a standard 28-day menstrual cycle and ovulation on day 14. It is the most widely used initial method because it doesn't require a specific date from a medical procedure.
The Logic: Pregnancy is considered to start from the first day of the LMP. A full-term pregnancy is typically 40 weeks (280 days) from this date.
Calculation: To calculate the EDD using Naegele's Rule:
Take the first day of your LMP.
Add 7 days to that date.
Subtract 3 months from the resulting date.
Add 1 year to that date.
Alternatively, and often simpler for calculators: add 280 days to the first day of your LMP.
Limitations: Naegele's Rule is most accurate for individuals with regular 28-day cycles. For those with longer or shorter cycles, or irregular periods, the EDD might be less precise.
Method 2: Using an Early Ultrasound Scan
An early pregnancy ultrasound, particularly one performed in the first trimester (before 13 weeks), is considered the most accurate way to date a pregnancy. This is because the size of the fetus is very consistent during this period.
The Logic: The size of the embryo or fetus measured during an early ultrasound (e.g., crown-rump length – CRL) correlates strongly with its gestational age.
Calculation: If you have a scan date and the gestational age measured at that scan, you can calculate the EDD by:
Determining the number of days from the scan date to the expected 40-week mark.
This is calculated by: (40 weeks – Gestational Age at Scan) * 7 days per week.
Add this number of days to the scan date.
For example, if a scan at 10 weeks and 3 days (10.5 weeks) was done on January 1st, 2024:
Remaining weeks: 40 – 10.5 = 29.5 weeks
Remaining days: 29.5 * 7 = 206.5 days. Since we can't add half days easily, and due dates are estimates, we typically round or consider the range. For practical purposes, let's calculate 207 days.
Add 207 days to January 1st, 2024.
This would lead to an EDD around August 5th, 2024.
Accuracy: First-trimester ultrasounds are generally accurate within +/- 5-7 days. Later ultrasounds become less precise for dating.
What is Gestational Age?
Gestational age is the term used to describe how far along a pregnancy is. It is typically measured in weeks and days. It is calculated from the first day of the woman's last menstrual period (LMP).
Full Term: A pregnancy is considered full-term between 37 weeks and 40 weeks and 6 days.
Premature: A birth before 37 weeks is considered premature.
Post-term: A birth after 42 weeks is considered post-term.
Using This Calculator
Enter the first day of your last menstrual period to get an EDD based on Naegele's Rule. If you have a date from an early ultrasound and know the gestational age at that scan, you can enter those details for a more precise EDD based on the ultrasound dating.
function calculateDueDate() {
var lmpDateInput = document.getElementById("lmpDate");
var eddScanDateInput = document.getElementById("eddScanDate");
var gestationalAgeInput = document.getElementById("gestationalAge");
var resultDiv = document.getElementById("result");
var lmpDateStr = lmpDateInput.value;
var eddScanDateStr = eddScanDateInput.value;
var gestationalAgeStr = gestationalAgeInput.value;
var lmpDate = lmpDateStr ? new Date(lmpDateStr) : null;
var eddScanDate = eddScanDateStr ? new Date(eddScanDateStr) : null;
var gestationalAge = gestationalAgeStr ? parseFloat(gestationalAgeStr) : null;
var edd = null;
var weeksFromLmp = null;
var daysFromLmp = null;
var weeksFromScan = null;
var daysFromScan = null;
var currentWeeks = null;
var currentDays = null;
var oneDay = 1000 * 60 * 60 * 24;
var daysInWeek = 7;
// Prioritize scan date if available and valid
if (eddScanDate && gestationalAge && gestationalAge >= 0) {
// Calculate days remaining from scan date to 40 weeks
// Gestational age is measured from LMP, so 40 weeks is the target.
// If scan age is 10.5 weeks, there are 40 – 10.5 = 29.5 weeks remaining.
var weeksRemaining = 40 – gestationalAge;
var daysRemaining = Math.round(weeksRemaining * daysInWeek);
var scanDateForCalc = new Date(eddScanDate);
edd = new Date(scanDateForCalc.getTime() + daysRemaining * oneDay);
// Calculate current pregnancy duration as of today
var today = new Date();
var diffInDaysFromLMPForScanDate = Math.round((scanDateForCalc.getTime() – lmpDate.getTime()) / oneDay);
var estimatedLMPfromScan = new Date(scanDateForCalc.getTime() – gestationalAge * daysInWeek * oneDay);
if (lmpDate && edd && estimatedLMPfromScan.getTime() === lmpDate.getTime()) {
// If LMP is provided and consistent with scan, calculate current status
var daysSinceLMP = Math.round((today.getTime() – lmpDate.getTime()) / oneDay);
currentWeeks = Math.floor(daysSinceLMP / daysInWeek);
currentDays = daysSinceLMP % daysInWeek;
} else if (edd) {
// If LMP is not provided or inconsistent, calculate from scan date
var daysSinceScan = Math.round((today.getTime() – scanDateForCalc.getTime()) / oneDay);
currentWeeks = Math.floor((gestationalAge * daysInWeek + daysSinceScan) / daysInWeek);
currentDays = (gestationalAge * daysInWeek + daysSinceScan) % daysInWeek;
}
var formattedEdd = edd.toLocaleDateString('en-US', { month: 'long', day: 'numeric', year: 'numeric' });
resultDiv.innerHTML = `
Estimated Due Date (via Scan): ${formattedEdd}
${(currentWeeks !== null && currentDays !== null) ? `Current Gestational Age: ${currentWeeks} weeks ${currentDays} days` : "}
`;
resultDiv.style.backgroundColor = '#d4edda'; // Success Green tint
resultDiv.style.color = '#155724';
resultDiv.style.borderColor = '#28a745';
} else if (lmpDate) {
// Calculate EDD using Naegele's Rule (add 280 days)
var lmpDateForCalc = new Date(lmpDate);
edd = new Date(lmpDateForCalc.getTime() + 280 * oneDay);
// Calculate current pregnancy duration as of today
var today = new Date();
var daysSinceLMP = Math.round((today.getTime() – lmpDate.getTime()) / oneDay);
currentWeeks = Math.floor(daysSinceLMP / daysInWeek);
currentDays = daysSinceLMP % daysInWeek;
var formattedEdd = edd.toLocaleDateString('en-US', { month: 'long', day: 'numeric', year: 'numeric' });
resultDiv.innerHTML = `
Estimated Due Date (via LMP): ${formattedEdd}
Current Gestational Age: ${currentWeeks} weeks ${currentDays} days
`;
resultDiv.style.backgroundColor = '#d4edda'; // Success Green tint
resultDiv.style.color = '#155724';
resultDiv.style.borderColor = '#28a745';
} else {
resultDiv.innerHTML = "Please enter at least the first day of your last menstrual period.";
resultDiv.style.backgroundColor = '#f8d7da'; // Danger Red tint
resultDiv.style.color = '#721c24';
resultDiv.style.borderColor = '#dc3545';
}
}