Enter your LMP and current gestational age to see results.
Understanding Gestational Age and Due Dates
Calculating gestational age and estimating a due date is a crucial part of prenatal care. It helps healthcare providers monitor the baby's growth and development, schedule necessary tests, and identify potential risks associated with preterm or post-term pregnancies. This calculator provides an estimate based on the standard method, which uses the first day of the Last Menstrual Period (LMP).
How Gestational Age is Calculated
Gestational age is typically calculated in weeks and days, starting from the first day of a woman's last menstrual period (LMP). This method assumes that ovulation and conception occurred approximately two weeks after the LMP, which is a standard biological timeframe for many women with regular 28-day menstrual cycles. The full-term pregnancy is considered to be 40 weeks (or 280 days) from the LMP.
The formula used is straightforward:
Total Gestational Days = (Current Date - LMP Date) in days Gestational Weeks = floor(Total Gestational Days / 7) Gestational Days = Total Gestational Days % 7
Alternatively, when you know the approximate current gestational weeks and days, you can use this information to project forward.
Estimating the Due Date (EDD)
The most common method for estimating the due date (also known as the Estimated Due Date or EDD) is Naegele's Rule. This rule adds 40 weeks (280 days) to the first day of the LMP.
The calculation for Naegele's Rule is as follows:
Take the first day of your LMP.
Add 7 days to that date.
Subtract 3 months from the resulting date.
Add 1 year to the resulting date.
For example, if your LMP started on October 10, 2023:
Add 7 days: October 17, 2023
Subtract 3 months: July 17, 2023
Add 1 year: July 17, 2024
So, the EDD would be July 17, 2024.
Our calculator simplifies this by directly calculating 40 weeks (280 days) from the LMP.
Why is Gestational Age Important?
Fetal Development Monitoring: Gestational age provides a standardized timeline to compare fetal growth against.
Risk Assessment: It helps identify pregnancies at risk for preterm birth (born before 37 weeks) or post-term birth (born after 42 weeks), both of which can have associated complications.
Medical Decisions: Gestational age guides decisions about prenatal screenings, tests, and potential interventions.
Parental Preparation: Knowing the estimated due date allows parents to prepare for the baby's arrival.
Important Considerations
This calculator provides an estimate. The actual date of birth can vary significantly.
The LMP method assumes a regular 28-day cycle. If your cycles are irregular, shorter, or longer, the LMP date might be less accurate.
An early ultrasound, especially in the first trimester, is often considered the most accurate way to determine gestational age.
Always consult with your healthcare provider for accurate dating of your pregnancy and personalized prenatal care.
function calculateGestationalAgeAndDueDate() {
var lmpDateStr = document.getElementById("lastPeriodStart").value;
var currentWeeks = parseInt(document.getElementById("gestationalWeeks").value);
var currentDays = parseInt(document.getElementById("gestationalDays").value);
var resultDiv = document.getElementById("result");
resultDiv.innerHTML = ""; // Clear previous results
if (!lmpDateStr) {
resultDiv.innerHTML = 'Please enter the start date of your Last Menstrual Period (LMP).';
return;
}
var lmpDate = new Date(lmpDateStr);
var today = new Date();
today.setHours(0, 0, 0, 0); // Normalize today's date to midnight
// Calculate gestational age based on LMP date and today's date
var diffTime = Math.abs(today.getTime() – lmpDate.getTime());
var diffDays LMP = Math.ceil(diffTime / (1000 * 60 * 60 * 24));
var lmpWeeks = Math.floor(diffDaysLMP / 7);
var lmpDays = diffDaysLMP % 7;
// Calculate due date based on LMP
var dueDate = new Date(lmpDate);
dueDate.setDate(lmpDate.getDate() + 280); // Add 280 days (40 weeks)
// Format the due date
var formattedDueDate = dueDate.toLocaleDateString('en-US', { year: 'numeric', month: 'long', day: 'numeric' });
// — Calculations based on user input for current weeks/days —
var calculatedDueDateFromInput = null;
if (!isNaN(currentWeeks) && !isNaN(currentDays) && currentWeeks >= 0 && currentDays >= 0 && currentDays <= 6) {
// Calculate the date corresponding to the current weeks/days input
var daysFromLMPByInput = (currentWeeks * 7) + currentDays;
var inputBasedCurrentDate = new Date(lmpDate);
inputBasedCurrentDate.setDate(lmpDate.getDate() + daysFromLMPByInput);
// Calculate due date based on this specific input date
calculatedDueDateFromInput = new Date(inputBasedCurrentDate);
calculatedDueDateFromInput.setDate(inputBasedCurrentDate.getDate() + (280 – daysFromLMPByInput));
var formattedCalculatedDueDate = calculatedDueDateFromInput.toLocaleDateString('en-US', { year: 'numeric', month: 'long', day: 'numeric' });
var weeksUntilDue = Math.floor((calculatedDueDateFromInput.getTime() – inputBasedCurrentDate.getTime()) / (1000 * 60 * 60 * 24 * 7));
var daysUntilDue = Math.round((calculatedDueDateFromInput.getTime() – inputBasedCurrentDate.getTime()) / (1000 * 60 * 60 * 24)) % 7;
var resultHTML = `
Based on your LMP (${lmpDate.toLocaleDateString('en-US', { month: 'long', day: 'numeric', year: 'numeric' })}):
Estimated Gestational Age: ${lmpWeeks} weeks ${lmpDays} days
Estimated Due Date (EDD): ${formattedDueDate}
For your input of ${currentWeeks} weeks ${currentDays} days:
Projected Due Date: ${formattedCalculatedDueDate}
Weeks until Due Date: ${weeksUntilDue} weeks ${daysUntilDue} days
`;
resultDiv.innerHTML = resultHTML;
} else {
// If current weeks/days are not valid, just show LMP-based results
var resultHTML = `
Based on your LMP (${lmpDate.toLocaleDateString('en-US', { month: 'long', day: 'numeric', year: 'numeric' })}):
Estimated Gestational Age: ${lmpWeeks} weeks ${lmpDays} days
Estimated Due Date (EDD): ${formattedDueDate}
Please enter valid current gestational weeks and days for projected due date calculation.
`;
resultDiv.innerHTML = resultHTML;
}
}