Calculate your estimated gestational age in weeks based on your Last Menstrual Period (LMP) or Estimated Due Date (EDD).
(Enter either LMP or EDD. If you enter both, LMP will be used.)
Estimated Gestational Age:
— weeks —
Understanding Pregnancy Dating
Pregnancy is typically measured in weeks, starting from the first day of your Last Menstrual Period (LMP). This method, known as gestational age, is the most common way healthcare providers date a pregnancy. While conception usually occurs about two weeks after your LMP, dating starts from the LMP for consistency.
A full-term pregnancy is considered to be 40 weeks from the first day of your LMP. The estimated due date (EDD) is calculated based on this 40-week period.
How the Calculator Works
This calculator uses two primary methods to estimate gestational age:
Using Last Menstrual Period (LMP): If you provide the start date of your LMP, the calculator determines the number of days between that date and the current date (or a specified reference date if not today) and divides it by 7 to get the number of weeks.
Formula:Weeks = (Current Date - LMP Date) / 7 days
(Approximately, if current date is today).
Using Estimated Due Date (EDD): If you provide your EDD, the calculator subtracts 40 weeks (280 days) from your EDD to estimate the LMP, and then calculates the weeks from that estimated LMP to the current date. Alternatively, it calculates the difference between the EDD and the current date and subtracts this from 40 weeks.
Formula:Weeks = 40 weeks - (EDD - Current Date) / 7 days
(Approximately, if current date is today).
Note: The most accurate dating is usually determined by an early ultrasound in the first trimester. This calculator provides an estimate based on the dates you provide. The "current date" used for calculation is the date the calculation is performed.
Why is Dating Important?
Accurately dating a pregnancy is crucial for several reasons:
Monitoring Fetal Development: It helps track your baby's growth and development against standard milestones.
Scheduling Prenatal Care: It determines the timing for important appointments, screenings, and ultrasounds.
Assessing Full-Term Status: It helps determine if a baby is born premature, on time, or post-term.
Making Informed Decisions: It's vital for planning and understanding the progression of your pregnancy.
Using the Calculator
Simply enter the start date of your LMP or your Estimated Due Date (EDD) into the respective fields. If both are entered, the LMP date will be prioritized for the calculation. Click "Calculate Weeks" to see your estimated gestational age.
function calculatePregnancyWeeks() {
var lmpDateInput = document.getElementById("lmpDate");
var eddDateInput = document.getElementById("eddDate");
var resultValue = document.getElementById("result-value");
var additionalInfo = document.getElementById("additional-info");
var lmpDate = lmpDateInput.value;
var eddDate = eddDateInput.value;
var currentDate = new Date();
currentDate.setHours(0,0,0,0); // Normalize to midnight for consistent date comparison
var startDate = null;
var calculationMethod = "";
if (lmpDate) {
var lmp = new Date(lmpDate);
lmp.setHours(0,0,0,0);
if (isNaN(lmp.getTime())) {
resultValue.textContent = "Invalid LMP Date";
additionalInfo.textContent = "";
return;
}
// Ensure LMP is not in the future
if (lmp > currentDate) {
resultValue.textContent = "LMP cannot be in the future";
additionalInfo.textContent = "";
return;
}
startDate = lmp;
calculationMethod = "Based on your LMP";
} else if (eddDate) {
var edd = new Date(eddDate);
edd.setHours(0,0,0,0);
if (isNaN(edd.getTime())) {
resultValue.textContent = "Invalid EDD Date";
additionalInfo.textContent = "";
return;
}
// Ensure EDD is not in the past (for accurate calculation)
// We can allow EDD in the past to calculate how far along *would have been*
// but for current weeks, it's less intuitive. Let's assume EDD is in the future or recent past for 'current' weeks.
// If EDD is significantly in the past, it might mean the input is wrong or the pregnancy has concluded.
// For simplicity, we'll calculate based on it, but add a note if it's far in the past.
var weeksDifferenceFromEdd = (edd.getTime() – currentDate.getTime()) / (1000 * 60 * 60 * 24 * 7);
var estimatedLmp = new Date(edd.getTime() – (40 * 7 * 24 * 60 * 60 * 1000)); // Subtract 40 weeks (280 days)
estimatedLmp.setHours(0,0,0,0);
startDate = estimatedLmp; // Use the estimated LMP to calculate weeks from LMP
calculationMethod = "Based on your EDD (approx. " + Math.round(weeksDifferenceFromEdd) + " days remaining)";
if (edd < currentDate) {
additionalInfo.textContent = "Note: Your EDD is in the past. This calculation shows estimated weeks relative to that EDD.";
} else {
additionalInfo.textContent = "";
}
} else {
resultValue.textContent = "Please enter LMP or EDD";
additionalInfo.textContent = "";
return;
}
if (!startDate) {
resultValue.textContent = "Could not determine start date";
additionalInfo.textContent = "";
return;
}
var timeDiff = currentDate.getTime() – startDate.getTime();
var weeks = Math.floor(timeDiff / (1000 * 60 * 60 * 24 * 7));
var days = Math.floor((timeDiff / (1000 * 60 * 60 * 24)) % 7);
// Adjust if calculation resulted in negative weeks (e.g., LMP is today or in near future, EDD is far future)
if (weeks 0 ? ", " + days + " days" : "");
additionalInfo.textContent = calculationMethod;
}