The journey of 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 standard way healthcare providers estimate your due date and track your baby's development. Our Pregnancy Calculator by Week helps you pinpoint your current stage of pregnancy and estimate your expected delivery date based on this widely accepted dating method.
How the Calculation Works:
Pregnancy dating is primarily based on the concept of gestational age, which is calculated from the first day of a woman's Last Menstrual Period (LMP). A full-term pregnancy is considered to be 40 weeks (or 280 days) from the LMP.
Due Date Calculation: The estimated due date (EDD) is calculated by adding 40 weeks (280 days) to the first day of your LMP. This is often done using Naegele's rule: LMP + 7 days – 3 months + 1 year.
Current Week and Day: If you know your LMP, the calculator determines the number of weeks and days that have passed since that date.
Using Current Gestational Age: Alternatively, if you know your current gestational age in weeks (e.g., from an early ultrasound), you can input this value along with your LMP. The calculator will then project your estimated due date and confirm your current stage.
It's important to remember that the estimated due date is just an approximation. Babies arrive when they are ready, and it's common for births to occur within a week or two before or after the EDD.
Why Use a Pregnancy Calculator?
Track Development: Knowing your week of pregnancy helps you understand the milestones your baby is reaching.
Prepare for Appointments: It allows you to anticipate important prenatal appointments and screenings.
Manage Expectations: Understanding your estimated due date can help you and your partner prepare for the arrival of your baby.
Informational Purposes: This calculator is a helpful tool for general information and planning. It is not a substitute for professional medical advice.
Always consult with your healthcare provider for accurate dating of your pregnancy and personalized advice.
function calculatePregnancyWeek() {
var lmpInput = document.getElementById("lastMenstrualPeriod");
var gestationalAgeInput = document.getElementById("gestationalAge");
var pregnancyWeeksOutput = document.getElementById("pregnancyWeeks");
var pregnancyDaysOutput = document.getElementById("pregnancyDays");
var dueDateOutput = document.getElementById("dueDate");
var lmpValue = lmpInput.value;
var gestationalAgeValue = parseFloat(gestationalAgeInput.value);
if (!lmpValue && isNaN(gestationalAgeValue)) {
alert("Please enter either your Last Menstrual Period date or your current Gestational Age.");
return;
}
var edd = null;
var weeksDifference = NaN;
var daysDifference = NaN;
var today = new Date();
today.setHours(0, 0, 0, 0); // Normalize today's date to midnight
if (lmpValue) {
var lmpDate = new Date(lmpValue);
lmpDate.setHours(0, 0, 0, 0); // Normalize LMP date to midnight
if (isNaN(lmpDate.getTime())) {
alert("Invalid date format for Last Menstrual Period.");
return;
}
// Calculate estimated due date (40 weeks or 280 days from LMP)
var eddCalc = new Date(lmpDate);
eddCalc.setDate(lmpDate.getDate() + 280);
edd = eddCalc;
// Calculate difference between today and LMP
var timeDiff = today.getTime() – lmpDate.getTime();
var totalDays = Math.floor(timeDiff / (1000 * 60 * 60 * 24));
weeksDifference = Math.floor(totalDays / 7);
daysDifference = totalDays % 7;
// Ensure daysDifference is positive if calculation results in negative
if (daysDifference < 0) {
daysDifference += 7;
weeksDifference -= 1; // Adjust weeks if days were negative
}
} else if (!isNaN(gestationalAgeValue)) {
// If only gestational age is provided, we can only estimate based on that
// We need a reference point for current date for this to make sense for due date
if (gestationalAgeValue 42) {
alert("Gestational age must be between 0 and 42 weeks.");
return;
}
weeksDifference = gestationalAgeValue;
daysDifference = 0; // Assuming exact weeks when only GA is given
// If we have current date, we can estimate EDD backwards
var eddCalc = new Date(today);
var daysToSubtract = (gestationalAgeValue * 7) + daysDifference;
eddCalc.setDate(today.getDate() – daysToSubtract + 280); // Add 280 days to get EDD from LMP equivalent
edd = eddCalc;
}
// Display results
if (!isNaN(weeksDifference) && !isNaN(daysDifference)) {
pregnancyWeeksOutput.textContent = weeksDifference + " weeks";
pregnancyDaysOutput.textContent = daysDifference + " days";
} else {
pregnancyWeeksOutput.textContent = "–";
pregnancyDaysOutput.textContent = "–";
}
if (edd instanceof Date && !isNaN(edd.getTime())) {
var day = edd.getDate();
var month = edd.getMonth() + 1; // Months are zero-indexed
var year = edd.getFullYear();
dueDateOutput.textContent = "Due Date: " + month + "/" + day + "/" + year;
} else {
dueDateOutput.textContent = "Due Date: –";
}
}