Understanding your pregnancy week is a key part of the journey, helping you track your baby's development and prepare for upcoming milestones. Our Pregnancy Week Calculator by Due Date provides an easy way to determine your current week of pregnancy based on your estimated due date.
How the Pregnancy Week Calculator Works
A full-term pregnancy is typically considered to be 40 weeks, or approximately 280 days, from the first day of your last menstrual period (LMP). While many women know their estimated due date from an ultrasound or their healthcare provider, it can sometimes be tricky to figure out exactly which week of pregnancy you are currently in.
This calculator simplifies that process. By inputting your estimated due date and today's date, it calculates the number of days remaining until your due date. It then subtracts these remaining days from the total 280-day pregnancy duration to determine how many days have already passed since the start of your pregnancy. This elapsed time is then converted into weeks and days, giving you your current pregnancy week.
It's important to remember that due dates are estimates, and only about 5% of babies are born exactly on their due date. This calculator provides an approximation to help you track your progress, but always consult with your healthcare provider for personalized medical advice and precise dating.
Using the Calculator
To use the Pregnancy Week Calculator, simply follow these steps:
Enter Your Expected Due Date: This is the date your doctor or ultrasound has estimated your baby will arrive.
Enter Today's Date: The calculator will automatically pre-fill this with the current date, but you can adjust it if you want to see your pregnancy week for a past or future date.
Click "Calculate": The calculator will instantly display your current pregnancy week and the remaining time until your due date.
Pregnancy Week Calculator by Due Date
document.addEventListener('DOMContentLoaded', function() {
var today = new Date();
var dd = String(today.getDate()).padStart(2, '0');
var mm = String(today.getMonth() + 1).padStart(2, '0'); //January is 0!
var yyyy = today.getFullYear();
document.getElementById('currentDateInput').value = yyyy + '-' + mm + '-' + dd;
// Set a default due date (e.g., 20 weeks from today for demonstration)
var defaultDueDate = new Date();
defaultDueDate.setDate(today.getDate() + (20 * 7)); // 20 weeks from today
var ddd = String(defaultDueDate.getDate()).padStart(2, '0');
var ddm = String(defaultDueDate.getMonth() + 1).padStart(2, '0');
var ddy = defaultDueDate.getFullYear();
document.getElementById('dueDateInput').value = ddy + '-' + ddm + '-' + ddd;
});
function calculatePregnancyWeek() {
var dueDateStr = document.getElementById('dueDateInput').value;
var currentDateStr = document.getElementById('currentDateInput').value;
var resultDiv = document.getElementById('result');
if (!dueDateStr || !currentDateStr) {
resultDiv.innerHTML = 'Please enter both the Expected Due Date and Today\'s Date.';
return;
}
var dueDate = new Date(dueDateStr);
var currentDate = new Date(currentDateStr);
if (isNaN(dueDate.getTime()) || isNaN(currentDate.getTime())) {
resultDiv.innerHTML = 'Please enter valid dates.';
return;
}
// Normalize dates to start of day to avoid time component issues
dueDate.setHours(0, 0, 0, 0);
currentDate.setHours(0, 0, 0, 0);
var totalPregnancyDays = 280; // 40 weeks
var timeDiffMs = dueDate.getTime() – currentDate.getTime();
var daysRemaining = Math.ceil(timeDiffMs / (1000 * 60 * 60 * 24));
var daysElapsed = totalPregnancyDays – daysRemaining;
var currentWeek = Math.floor(daysElapsed / 7);
var daysIntoCurrentWeek = daysElapsed % 7;
var weeksRemaining = Math.floor(daysRemaining / 7);
var daysRemainingInWeek = daysRemaining % 7;
var output = ";
if (daysElapsed 42) {
output = 'You are past your estimated due date. Please consult your healthcare provider.';
output += 'You are approximately ' + (currentWeek – 40) + ' week(s) and ' + daysIntoCurrentWeek + ' day(s) past your due date.';
} else {
output = 'You are approximately in Week ' + currentWeek + ' and ' + daysIntoCurrentWeek + ' day(s) of your pregnancy.';
output += 'You have approximately ' + weeksRemaining + ' week(s) and ' + daysRemainingInWeek + ' day(s) remaining until your due date.';
}
var conceptionDate = new Date(dueDate.getTime());
conceptionDate.setDate(conceptionDate.getDate() – 266); // 38 weeks = 266 days from conception to due date
var lmpDate = new Date(dueDate.getTime());
lmpDate.setDate(lmpDate.getDate() – 280); // 40 weeks = 280 days from LMP to due date
var options = { year: 'numeric', month: 'long', day: 'numeric' };
output += 'Your estimated conception date was around: ' + conceptionDate.toLocaleDateString('en-US', options) + '';
output += 'Your estimated Last Menstrual Period (LMP) start date was around: ' + lmpDate.toLocaleDateString('en-US', options) + '';
resultDiv.innerHTML = output;
}
Example Calculation
Let's say your estimated due date is March 26, 2024, and today's date is October 26, 2023.
Total Pregnancy Duration: 280 days (40 weeks)
Days Remaining until Due Date: Approximately 152 days (from Oct 26, 2023, to Mar 26, 2024)
Days Elapsed since Start of Pregnancy: 280 days – 152 days = 128 days
Current Pregnancy Week: 128 days / 7 days/week = 18 weeks and 2 days
Weeks Remaining: 152 days / 7 days/week = 21 weeks and 5 days
Based on these inputs, the calculator would tell you that you are approximately in Week 18 and 2 days of your pregnancy, with about 21 weeks and 5 days remaining until your due date.
Important Considerations
Due Date Accuracy: Due dates are estimates. Ultrasounds in the first trimester are generally the most accurate for dating a pregnancy.
Individual Variation: Every pregnancy is unique. Some babies arrive earlier or later than their due date.
Medical Advice: This calculator is for informational purposes only and should not replace professional medical advice. Always consult your doctor or midwife for any health concerns or questions about your pregnancy.